A. Dolphins can be trained to assist divers in ocean rescues.

We are concerned more with communication skills and not non-communication skills.

B. Gorillas in captivity often learn hand signals for food and water.

Gorillas are higher order primates – the argument is about low order ones.

C. Dogs are capable of sensing their owners' moods and often exhibit concern if they sense sadness.

True – this strengthens the conclusion by providing another example of how lower order animals communicate

D. Chimpanzees can memorize long sequences of key punches on machines that dispense food.

Chimps are higher order primates – the argument is about low order ones.

E. Alex does not exhibit aggression when offered a gesture that he specifically requested.

This repeats the example on parrots. A different animal is more preferable as an example to buttress the conclusion.

I chose C too but answer in not C.

I chose C first but I think E seems to be the best answer...

Some animals, such as dolphins, dogs, and African grey parrots, seem to exhibit cognitive functions typically associated with
higher-order primates such as chimpanzees, gorillas, and humans. Some parrots, for example, have vocabularies of hundreds of
words that they can string together in a comprehensible syntax. This clearly shows that humans and primates are not the only
animals capable of using language to communicate. One parrot, named Alex, has been known to ask to be petted or kissed and will
exhibit aggression if the gesture offered is not the specific one requested.

Which of the following, if true, would most strengthen the conclusion above?

A. Dolphins can be trained to assist divers in ocean rescues.

Nothing that suggests the ability to assist. Irrelevant. In addition the argument is about lower level primates and not high level primates

B. Gorillas in captivity often learn hand signals for food and water.
the argument is about lower level primates and not high level primates.

C. Dogs are capable of sensing their owners' moods and often exhibit concern if they sense sadness.

the argument is about lower level primates and not high level primates. Even though it talks about emotions, the conclusion is that are lower level animals
that belong to the category of high level primates.

D. Chimpanzees can memorize long sequences of key punches on machines that dispense food.

Irrelevant

E. Alex does not exhibit aggression when offered a gesture that he specifically requested.

Alex is a parrot (lower level primate) that knows a number of words. But in addition to that it displays this trait of not exhibiting
aggression when offered a gesture specifically requested by him. So he understands what kind of emotions he should display which is similar to higher level
primates.

This is a strenthen question. Strength questions typically confirm the assumption that links the evidence with the conclusion.

Evidence: Alex will exhibit aggression if the gesture offered is not the specific one requested.

Conclusion: based on Alex's example, parrots are like human who communicate with a language.

Assumptions:
- exhibiting aggression in this case indicate communication via language
- this example is representative of all parrots or its group of animals
- this evidence is valid and relevant. That is, Alex has exhibited aggression BECAUSE the gesture offered is not the specific one requested.

Answer : E

Surprisingly, all other answer choices were either out-of-scope or don't help at all with the assumption thing.

I thinks its an E.