Answer (B) is incorrect because it is too strong. We are told throughout the passage that neutrinos may be dark matter, but that they could only account for 20% of it. But, we are also told that neutrinos are only one hypothesis (line 18-20), and that numerous ones have been proposed. Thus, we don't know that other theories are incapable of explaining dark matter.
Answer (D) is correct because it points out an underlying assumption in the argument. The first paragraph discusses gravitation and tells us that it is "believed to shape the structures" of the universe (lines 5-7). The passage then goes on to explain an issue with gravitation (aka that the mass isn't accounted for) and a possible solution to that issue (neutrinos). This is all based on the idea that gravitation is the only force behind the organization of the universe.
