The question asks the following:

The view taken by the researchers concerning the monogamy of tamarins depended
on a questionable assumption. Which of the following could have served as that assumption?

So, our first step is to find "The view taken by the researchers concerning the monogamy of tamarins."

We find that view in the last sentence of the passage:

tamarins were viewed as monogamous

To find the questionable assumption that the view depended on, we have to find in the passage the researchers' basis for that view.

We find the following:

On the basis of the fact that breeding occurred, tamarins were viewed as monogamous.

So, the basis for the view that tamarins are monogamous was simply "the fact that breeding occurred."

Now, let's consider the answer choices.

(A) The suppression of fighting between related females serves to protect their common genetic inheritance.

This choice tries to tempt us with a connection to what the passage says about "fights ... between unrelated females." So, we have to be careful not to choose this choice just because it's connected to the passage in that way.

The researchers didn't have to assume that the suppression of fighting between related females has this effect to conclude that tamarins are monogamous. After all, tamarins could be monogamous even if the suppression of fighting between related females did not have this effect.

(B) Adult male tamarins contribute to the care of tamarin infants.

This choice could be a reason for monogamy. Perhaps tamarins are monogamous because adult male tamarins contribute to the care of infants.

However, we are not looking for a why tamarins are monogamous. We are looking for an assumption that the view that tamarins are monogamous depended on, and the researchers didn't have to assume that male tamarins do this specific thing to conclude that tamarins are monogamous.

(C) The social system of tamarins requires monogamous pairing.

This choice is tempting because it's closely related to the view that tamarins are monogamous. Notice, however, that the researchers didn't have to assume that monogamy is required by the social system of tamarins to conclude that tamarins are monogamous. After all, tamarins could be monogamous even if their social system doesn't require monogamous pairing.

(D) Male tamarin monkeys do not display aggressive behavior in the wild.

This choice tries to tempt us with a connection to the aggressive behavior of female tamarins described in the passage. At the same time, this choice is clearly not an assumption that the view taken by the researchers depended on since tamarins could be monogamous regardless of whether male tamarins display aggressive behavior in the wild.

(E) The way the tamarins were kept in cages did not affect their mating behavior.

To see whether this choice is correct, we can return to the passage to see what it says about "the way the tamarins were kept in cages."

We find the following:

Tamarins were kept caged in male-female pairs

Combining this information with the information we already have about the view of the researchers, we get the following.

Tamarins were kept caged in male-female pairs ... On the basis of the fact that breeding occurred, tamarins were viewed as monogamous.

We see that the researchers based their view on the actions of tamarins "kept caged in male-female pairs." So, their view that tamarins are monogamous depended on the assumption that what they observed didn't occur only because the tamarins were kept in cages. Thus, we see that the view of the researchers depended on the assumption that "The way the tamarins were kept in cages did not affect their mating behavior."

The correct answer is (E).