Some animals, such as dolphins, dogs, and African grey parrots, seem

I don't know the source, but my understanding is that the OA is (E).

The argument takes for granted that higher-order primates (chimpanzees, gorillas, and human) have higher cognitive function, including language. The argument asserts that many other animals, including parrots and possibly dogs & dolphins, also have the ability to use language to communicate. That phrase is key to understanding this question.

First of all, choices (B) & (D) add further examples of higher-order primates doing sophisticated thinking. No matter how sophisticated the thinking of higher-order primates, this tells us zilch about whether animals outside of the higher-order primates have any sophisticated abilities. These are irrelevant.

Choice (A) is deceptive. As you may know from outside sources, dolphins are pretty clever. If this had been an example of dolphins doing something very sophisticated, then that could be a strengthener. As it happens, many many animals (even insects!!) can be trained to do things. It's not at all clear whether being trained to assist in ocean rescues would involve very sophisticated skills or tremendously easy things to do (e.g. tow someone or something to the surface). Because the nature of this activity is not clear, we cannot say this is a clear strengthener, so it cannot be correct.

Choice (C) is about emotions. The abilities to feel emotions and to sense emotions in others are not cognitive abilities at all. Yes, the ability to talk about emotions, or write poetry evoking emotions, or something such as that --- those involve cognitive skills, but simply feeling emotions or sensing emotions is not cognitive at all. That's why (C) is irrelevant.

This only leave (E). Think if this were false. If the parrot Alex were always aggressive, then the aggression would have nothing to do with what he said in words, and this would destroy the only solid piece of evidence in the argument. The only way Alex's aggression to an unrequested gesture would make sense as a piece of evidence would be if he show no aggression to a requested gesture. Therefore, (E) is a clear strengthener --- it reinforces one assumption of the argument.

Overall, this is a good question, although it may demand an awfully sophisticated understanding of either psychology or cognitive science --- a little more background than the GMAT would typically expect.

Does all this make sense?
Mike