If reptiles were capable of complex reasoning, they would sometimes be able to make major changes in their behaviour. As they are not doing that therefore they are unable to make complex reasoning.

(D)

A is more like an extreme generalization about animal reasoning with an "if only" condition.
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For example, "He hasn't taken his GMAT yet, it must be true that he isn't in a Bschool."

It would be equivalent to say "if he is in a Bschool he must have taken the GMAT".

Choice A would be equivalent to say "He has taken his GMAT only if he is in a Bschool". Which clearly may not be right because he may have taken GMAT and still not be in a bschool yet.
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(This is my first post. I teach LSAT, GMAT and GRE prep at Kaplan, and one of my students told me about GMATClub.)

Honghu's analysis is correct. This kind of question appears more often on the LSAT than the GMAT; it uses the formal logic concepts which Honghu explains.

The evidence is that reptiles are unable to make major alterations in their behaviour; the conclusion is that they are not capable of complex reasoning. As most posters have recognized, this reasoning assumes that IF an animal can't make major alterations in its behaviour, THEN it is not capable of complex reasoning.

This assumption is an "If X, then Y" statement, and so the principles of formal logic apply. Most importantly, knowing that "If X, then Y" absolutely does NOT prove "If NOT X, then NOT Y". And equally, knowing that "If NOT X, then NOT Y" does NOT prove "If X, then Y".

In this particular case, the assumption is what we just said: IF an animal canNOT make major alterations in its behaviour, THEN it is NOT capable of complex reasoning. The critical thing to understand is that this "if-then" statement does not tell us ANYTHING about what is true if an animal CAN make major alterations in its behaviour. If that is true, this statement does NOT tell us whether or not it is capable of complex reasoning. It might be, or it might not be: The statement simply doesn't give us any information about that situation.

Another way of describing what the assumption says is this: NOT being able to make major alterations in behaviour is SUFFICIENT to show that the animal is NOT capable of complex reasoning. But it is not NECESSARY: an animal could be NOT capable of complex reasoning while still being ABLE to make major alterations in behaviour.

That should explain why we the assumption "If it canNOT X, then it is NOT Y" absolutely does NOT mean "If it CAN X, then it IS Y". Equating these two statements is the oldest logical error known to civilization; the Romans called it the "pons asinorum". The GMAT uses it sometimes, and the LSAT uses it over and over and over again.

What CAN be equated to an "if-then" statement is the contrapositive of that statement:

Statement: If X, then Y
Contrapositive: If NOT Y, then NOT X

As Honghu says, "We know that not M => Not C is equivalent to C=>M." That's the contrapositive. To say the same thing a little more slowly: The assumption is "If it canNOT X, then it is NOT Y". The equivalent to this statement is "If it IS Y, then it CAN X" -- NOT, repeat not, "If it can X, then it is Y".

So the correct answer choice must say either "If it cannot X, then it is not Y" (the original assumption) or "If it is Y, then it can X" (the contrapositive, which is equivalent). Answer choice D provides the contrapositive, so it is correct.

Why is A wrong? Because of one word. At first glance, it SEEMS to say the same thing as D: If reptiles were capable of complex reasoning, then they could make changes in their behaviour. In reality, it does not say this at all. Why? Because instead of "if", the sentence contains "only if". For the purposes of logical reasoning, the phrase "only if" does NOT mean "if"; it actually means "then". So A actually says that if an animal can make alterations in its behaviour, THEN it must be capable of complex reasoning -- which is exactly what we canNOT conclude from the original assumption
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Some psychologists attribute complex reasoning ability to reptiles, claiming that simple stimulus-response explanations of some reptiles’ behaviors, such as food gathering, cannot account for the complexity of such behavior.
But since experiments show that reptiles are incapable of making major alterations in their behavior, for example, when faced with significant changes in their environment,
Conclusion:
these animals must be incapable of complex reasoning.

[Significant EC -> Major AB => Capable of CR].

Which one of the following is an assumption required by Sylvia’s argument?

(B) Simple stimulus-response explanations can in principle account for all reptile behaviors. [nope]
(C) Reptile behavior appears more complex in the field than laboratory experiments reveal it to be. [puts the arg in doubt.Leave it]
(E) Complex reasoning and responses to stimuli cannot both contribute to the same behavior [nope]

A Vs D:
(A) Animals could make major changes in their behavior only if they were capable of complex reasoning.
>>
1. Animals here is used in general. Whereas argument is concerned with reptiles only.
2."Only if" too restrictive, whereas argument is concerned abt some behaviors.
(D) If reptiles were capable of complex reasoning, they would sometimes be able to make major changes in their behavior.

Below is the official explanation from Veritas.

Solution: D

This is a Strengthen question, as demonstrated by the phrase, “which one of the following is an assumption.” Therefore, our first item of business is therefore to identify the argument’s conclusion and then look for gaps between the conclusion and the premises. The conclusion for this particular argument is at the end of the passage: “[reptiles] must be incapable of complex reasoning.” There are two clear, textual reasons why this statement is the conclusion. First, this statement uses the phrase, “must be.” This is a value judgment, one of the signs we may be dealing with a conclusion. Another reason we can pick out this conclusion in the midst of everything else is the word “since.” “Since” is a type of conclusion tag, but it attaches itself to a premise immediately next to the conclusion. Order is crucial. Thus, if we see the phrase “Since A, then B”, A is a premise supporting the conclusion, B. If we see the phrase “A, since B”, A is now the conclusion, and B is a premise undergirding that conclusion. The last sentence in the passage uses the structure, “Since A, then B.” Thus, the ending statement, “these animals must be incapable of complex reasoning,” must be the conclusion. The logical gap for this argument is not solely found in the conclusion, but in the “since” statement preceding the conclusion. As mentioned above, “since” is attached to a statement of fact required for the conclusion to be valid. However, the notion of animals being incapable of changing their behavior is not found anywhere else in the argument. The experiment supposedly implies that if an animal could demonstrate the capacity of making major behavioral changes, this would be “proof” of complex reasoning, but it leaves a large hole: could there be other possible evidences of complex reasoning besides the capacity for behavioral change? Just because there is no evidence of reptiles’ ability to make major behavioral changes doesn’t mean they can’t demonstrate complex reasoning in another manner. This is our logical gap. The correct answer must help us bridge this hole in the argument.

Answer choice “A” is a popular answer choice, but it actually fails to address the logical gap. The reason behind this is subtle, but can easily be discerned by comparing answer choice “A” to answer choice “D”. “A” tells us that the “only” possible way that an animal could have the capacity of making major changes in behavior is if the animal had a capacity for complex reasoning. In essence, major changes in behavior are “proof” of complex reasoning. However, notice that this still leaves open the possibility for other possible “evidences” of complex reasoning, without the need for demonstrating the capacity of making major changes in behavior. (Yes, demonstrating major changes in behavior is “proof” of complex reasoning, but is it the only proof? “Writing a sonnet” could be proof of complex reasoning, even if the creature never demonstrates major changes in behavior.) Thus, answer choice “A” actually reinforces the same faulty logic as that found in the experiment: “A” leaves open the possibility that reptiles could have complex reasoning in spite of not being able to show the capacity for major behavioral changes. It contains the same logical gap as the original.

Answer choice “B” is another clever trap by the Testmaker. In this case, “B” undermines the argument made by the psychologists quoted the beginning. However, undermining someone else’s argument is not the same as strengthening your own argument. The question explicitly asks us to identify the assumption in Sylvia’s argument – in other words find the statement of evidence that plugs the logical hole. Because answer choice “B” does not focus on the problem with Sylvia’s argument (in other words, it doesn’t help us bridge the gap between the capacity to make major changes and the capacity for complex reasoning), “B” cannot be the right answer.

Answer choice “C” actually undermines Sylvia’s argument. The basis for her argument is the evidence collected by a series of experiments. Because “C” claims that the results of such experiments fail to capture what is actually going on in terms of behavior, this would make any conclusions based off these experiments less solid.

Answer choice “D” successfully bridges the logical gap, though the Testmaker tries to conceal the correct answer by including the fuzzy, non-specific word “sometimes.” statement – “sometimes” is certainly weaker than “always” – answer choice “D” is the only answer that nudges us in the right direction. Remember: “Strengthen” questions are not “Prove” questions – we are looking for the answer that “most strengthens” the existing argument. The hole in Sylvia’s argument is the lack of linkage between the capacity to make major behavioral changes and the capacity for complex reasoning. Answer choice “D” states that if reptiles had complex reasoning, they would sometimes demonstrate the capacity for major behavioral changes. In essence, the capacity for complex reasoning “causes” the capacity to make major changes in behavior. While other causes of major changes in behavior could still exist independently of complex reasoning, if the capacity for major changes in behavior is absent, then there cannot be complex reasoning. To draw an analogy here, take the statement, “MBA grads must have taken the GMAT.” For the sake of argument, let us say this statement is true. Yes, other people might take the GMAT. But if you didn’t take the GMAT, this means, at bare minimum, you definitely did not get an MBA (since a necessary part of the “MBA-achieving” process is taking the GMAT.) In like manner, answer choice “D” tells us that if an animal has the capacity for complex reasoning, it must be able to change its behavior. Yes, there may be other reasons why the animal could change behavior. But if it can’t change behavior, this means, at bare minimum, the animal does not have the capacity for complex reasoning (since complex reasoning. While this is a very weak would “cause” a creature to be able to make major behavioral changes.) Answer choice “D” plugs the gap.

Lastly, answer choice “E” also fails to address the logical gap. The hole in Sylvia’s argument is the lack of linkage between the capacity to make major behavioral changes and the capacity for complex reasoning. Answer choice “E” does not bridge this gap, but instead focuses on “responses to stimuli” – something found in the argument made by some psychologists (quoted by Sylvia at the beginning.) However, the question explicitly asks us to identify the assumption in Sylvia’s argument – in other words, we must find the statement of evidence that plugs the logical hole. Because answer choice “E” does not focus on the problem with Sylvia’s argument, “E” cannot be the right answer. It doesn’t help us bridge the gap between the capacity to make major changes and the capacity for complex reasoning.

A is not the assumption because if it states a necessary condition as the only necessary condition.
The stimulus states

Reptiles are NOT modifying behaivour -> Reptiles are incapable of complex reasoning. Therefore capability of complex reasoning is a NECESSARY condition for behaviour modification but NOT the ONLY necessary condition.