Explanation

What we have here is a "strengthen the conclusion" question. To strengthen a conclusion, you need to make one of the argument's underlying assumptions into a supporting premise by stating it explicitly. This argument is essential:

1. Crowding leads to aggression in rats.
2. However, crowding does NOT lead to aggression in rhesus monkeys.
3. Therefore, crowding probably does NOT lead to aggression in monkeys the way it does in rats.

Statements 1 and 2 are premises. Statement 3 is our conclusion. The question you want to ask yourself is: "What assumption are they making in between statements 2 and 3?" That assumption is going to be the answer to this type of question.

In this example, you can't assume that the behavior of Rhesus monkeys would reflect the behavior of other monkeys. Although we could probably justify this statement if we watch a lot of Discovery Channel, we can't use that external knowledge on the GMAT. Answer choice C is a version of this assumption; it makes explicit the fact that rhesus monkeys are actually more aggressive than other monkeys. The natural next step of this chain of logic is that if crowding doesn't make rhesus monkeys act aggressively, it probably doesn't make any other monkeys act aggressively.

As a result, C is the best way to strengthen this conclusion