Thanks for the question! First, the conclusion here is the last sentence. (B) says that the argument has to assume that it’s to the advantage of some individuals that they be concerned with contributing to societal good. Let’s use the negation test. The opposite of “some” is “none.” The negation is thus “it’s to the advantage of no one that they be concerned with contributing to societal good.” Well, if that’s true, the argument falls apart. Because if it’s not to anyone’s advantage, then it can’t be damaging to them to not contribute. So negating (B) makes the argument fall apart, and so (B) is the assumption we want.
(A) is wrong because the argument’s conclusion is just about modern literature damaging individuals and society. So comparing modern individuals to those from earlier eras is irrelevant to this conclusion and so can’t be the assumption we’re looking for.
Hope this helps! Feel free to ask any other questions that you might have.