Let's start off with why (D) is correct, starting with the first passage. We can see the usage of the second-person "you" throughout, such as in lines 30-31 when the author writes "it could all be in your head." The general focus on weightlifting suggests that the passage isn't for a general audience; its readers are more interested in weightlifting specifically, just as bodybuilders would be. Passage B, on the other hand, cites more scientific studies and talks about weightlifting more in the third person, as in the first paragraph when the author refers to people who have been buff before. This is a contrast from the "you" used in passage A.
Now as for why (A) is wrong, note that "the phenomenon under discussion" is that of weightlifters being able to gain weight more rapidly the second time around. No one doubts this phenomenon specifically; what there is debate about is the reasons for it, and so the analysis of passage A is wrong. We could stop the analysis right here, as if the answer is partially wrong, it is completely wrong. But if we wanted to look at passage B, if it were talking to weightlifters as the first article did, I think we would expect a tone that appeals to their experiences more, rather than relying solely on scientific studies. Hope this helps!