The stimulus here is dense and technical, with a lot going on to distract us from the core argument, so let's start by breaking down what's going on. We have a researcher who's talking about chemicals called pheromones. What do we know about pheromones? (1) both humans and animals produce them; (2) some animal behavior is determined by pheromones; (3) "it is clear" that human behavior is not controlled by pheromones; (4) this is because of our ability to choose, i.e. free will; (5) for humans, pheromones are just an evolutionary throwback.

That's a lot of information, and not all of it ties together in the support structure of an argument. So let's work out which statements support each other, and from there we can dramatically clarify what's going on. On closer examination, the researcher's claims focus on humans, not animals—all of the statements about animals are just used as context. So we're left with claims (3), (4), and (5). Between these claims, only two of them seem to involve support: (4) supports (3), while (5) is just kind of thrown out there for emphasis. So claims (3) and (4) make up the actual argument.

Having cut through all the noise, then, here's our argument: the researcher's conclusion is that psychological factors, not chemical pheromones, determine human behavior. The only support offered is that humans are able to choose how they behave thanks to their free will.

Since we did all the work of simplifying that argument, it might be easier to see where the researcher has to make an assumption. Between the premise and conclusion, the researcher kind of shifts the topic of discussion: the premise discusses free will and choosing our behavior, while the conclusion is about pheromones not determining how we behave. So we can just kind of slot an implicit condition in there: if humans choose how they behave, then pheromones don't affect their behavior. The argument isn't perfectly conditional, so the correct answer might not look exactly like this, but it will be something along those lines. We need to bridge the gap between free will and pheromones not being involved in behavior. It's still important to stay open to other possibilities though, and to be careful in assessing the answer choices.