(A) is correct because we can see that throughout the passage, in each of the explanations, the two elements mentioned in (A) are brought up. Looking at the concluding lines of the passage, which sum it up and bring attention to what the author wants our takeaway to be, we see these two particular elements mentioned explicitly in lines 48-52.
(D) is incorrect because even if the different explanations for how mirrors work are up for debate, the main point is not to show that this debate exists. Otherwise, the passage would have focused more on points of contention between the two, reasons to believe one or the other, or how the debate has developed historically. Instead, the passage simply presents the two explanations, and so (D) cannot be the main point. Hope this helps.
