A dog enthusiast took home two puppies from the dog shelter. He fed on

This is a good point! However, the reason D seems tempting here is because you're mistaking "weaken" for "disprove."

The facts in D makes the author's comparison an invalid one--he did not give the two dogs equivalent amounts of food, so cannot logically draw a conclusion based on his findings. You are absolutely correct that it is still possible that he gave less generic food out, and the generic dog still got bigger. However, that possibility has become significantly more remote with the addition of the information found in D. And making the conclusion significantly more remote, even if it's still conceivable, is enough for the GMAT.

+1C

Premise - A dog enthusiast took home two puppies from the dog shelter. He fed one of the puppies super premium canned dog food, while he fed the other a generic brand from the grocery store. The dog fed on the generic brand gained weight twice as fast as the dog fed the super premium brand. Because of the difference in these results

Conclusion - that the generic brand actually provided nutrition superior to that of the super premium brand.

Any option which weakens the conclusion is our answer

A. A dog will sometimes gain more weight when eating inferior quality food because he must eat more of it to obtain the nutrients he needs. (Weakens)
B. The dogs were of mixed breeds and appeared to be descended from dogs of different sizes. (Weakens)
C. Both dogs ate all of the food given to them at each serving. (this has no effect on the conclusion and is thus our answer)
D. The dog enthusiast did not give the two dogs equal amounts of their respective foods. (Weakens)
E. The dog who received the super premium dog food suffered from a digestive system disorder that hindered his growth. (Weakens)

A. A dog will sometimes gain more weight when eating inferior quality food because he must eat more of it to obtain the nutrients he needs.
This clearly weakens

B. The dogs were of mixed breeds and appeared to be descended from dogs of different sizes.
This will weaken the argument as it says dogs will grow to different sizes not because of food but coz of genes

C. Both dogs ate all of the food given to them at each serving.
This somehow supports one of the many assumptions made by author that both ate same amounts for the time period

D. The dog enthusiast did not give the two dogs equal amounts of their respective foods.
This will weaken the assumption stated above

E. The dog who received the super premium dog food suffered from a digestive system disorder that hindered his growth.
This says the second dog did not grow because of some disease and not coz of food

Not quite following the reasoning on this one. On E we could stablish that the superior food caused the digestive disorder thus supporting the statement of the author. And correct answer C is not telling us how much food was given to the dogs ...say brand generic food was only 3kg a day and superior food was only 100 gr a day that would seriously support nutrition qualities on the superior brand.

No, it cannot be established from option E that the food caused the digestive disorder. This option merely states that the dog which grew less had a digestive disorder. It does not say how the disorder was caused. Because the dog had a digestive disorder, it grew less - even if it were fed generic brand, the digestive disorder would still have hindered it's growth.

Option C does not weaken or strengthen the argument. It is a neutral statement with no impact on the argument. The fact that both the dogs ate all the food does not imply that one of them was given less food. Hence option C does not weaken the argument and hence the correct answer.

A dog enthusiast took home two puppies from the dog shelter. He fed one of the puppies super premium canned dog food, while he fed the other a generic brand from the grocery store. The dog fed on the generic brand gained weight twice as fast as the dog fed the super premium brand. Because of the difference in these results, the dog enthusiast concluded that the generic brand actually provided nutrition superior to that of the super premium brand.

Each of the following, if true, would weaken the evidence for the dog enthusiast’s conclusion except for which of the following?

A. A dog will sometimes gain more weight when eating inferior quality food because he must eat more of it to obtain the nutrients he needs.
clearly weakens the conclusion since one dog had to eat more to get equal nutrition.

B. The dogs were of mixed breeds and appeared to be descended from dogs of different sizes.
weakens as the dogs are of different breed/type and different breed have different eating habits or have different appetite.

C. Both dogs ate all of the food given to them at each serving.
does not weaken but actually strengthen the conclusion because it cancels out the possibility that the dogs may not have finished their food respectively. Thus the difference lead to the different growth.

D. The dog enthusiast did not give the two dogs equal amounts of their respective foods.
weakens as it attacks the conclusion that only different type of food lead to different growth.

E. The dog who received the super premium dog food suffered from a digestive system disorder that hindered his growth.
weakness as it gives alternate explanation to why one dog grew lesser than another.

Conclusion: the generic brand actually provided nutrition superior to that of the super-premium brand.

A. A dog will sometimes gain more weight when eating inferior-quality food because he must eat more of it to obtain the nutrients he needs.
=> so because the dog eats generic food had to eat a higher amount of food, its weight increased higher. It is not because generic food provided higher nutrition => Provide another reason which attacked the conclusion
B. The dogs were of mixed breeds and appeared to be descended from dogs of different sizes.
=> Provide another reason which attacked the conclusion
C. Both dogs ate all the food given to them at each serving.=> Correct choice
D. The dog enthusiast did not give the two dogs equal amounts of their respective foods.
=> For example, if the dog fed on generic brand food was given a higher amount of food than that of the đo fed on super-premium brand, its weight can be higher
E. The dog who received the super-premium dog food suffered from a digestive system disorder that hindered his growth.
=> It is because of the digestive system which caused the lower weight of dog fed on super-premium brand, so it cant be said that the generic brand provided higher nutrition - Provide another reason which attacked the conclusion

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