(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