I recently had a student ask me about this question, so I thought I'd post my solution on the forum
This is an assumption question. With assumption questions, I always pick an answer that supports my conclusion and that is wtihin the scope of the passage (being careful not to be too trigger happy about using the idea of scope). I do this after I've figured out the core of the argument.
Here, I'd say the core of the argument is:
The behavior of macaques changed over time -> thus they are not completely captive to their genes.
Let's look at each answer choice.
(A) is certainly relevant, but this goes against the conclusion. This would effectively point out a way that all the facts could be true but that the macaques would still be beholden to their genes.
(B) Bingo! Not only is this the opposite of (A), which makes it look really good, it makes sense that this is what missing. The author points to changes that took place in a short period of time to indicate that they were probably not genetic. This assumption makes that clear - the author is assuming or should assume that changes that occur over a short period of time are probably not genetic or at least are not necessarily genetic
(C) is not really helpful or hurtful. We might be able to make the link from "regularly" in the passage to "typical" in the answer choice, but so what. This just tells us that "typical" is a *necessary* condition to show a genetic change, but that does not mean it is a *sufficient* condition to show genetic change.
(D) is pretty extreme. And in a way it's circular. If you assume that social behavior is never genetic where's the need to prove it? Additionally, "never" is a lot stronger than we really need for this passage.
(E) is totally irrelevant. The author is not making any claims or relying on any predictions about the future. He is working off what happened in the past.
Questions or comments from forum users are always appreciated