The black swan as mentioned in passage A is something that is believed by popper to be enough to show that a theory is incorrect. So for this question we are looking for an analogous piece in passage B - something that acts as a mechanism to completely discredit a theory.
How did Newton's theory get rejected? It tried to predict the orbit of Mercury, but was incorrect. The theory of general relativity comes along and lead to the rejection of Newtons theory. In other words, Mercury acted as the black swan -- showing that Newtons theory could not account for something (much like the black swan shows the theory that all white swans are..white).
Therefore, Mercury, much like the black swan, is what we can deduce is the catalyst for a theory being overthrown. Thus, the correct answer is (A).
Neptune is not correct because it actually confirms Newtons theory, NOT discredits. The black swan is negative evidence (mercury) not positive evidence (neptune)