I can see how this is a tricky one. The problem lies with linguistics and English language usage. At first glance, it appears that (A) neither entails nor presupposes (B), because Mary could very well be in New York at some point in time but that doesn't necessarily mean she is there now. Specifically, the "if"—as Doug Spoonwood points out—can be used to imply that the following clause may or may not be true. The trick is that the same sentence in English can also be used to imply the current state of something. For example, if I were to say, "If my mom found out I am not really going to school, she would be angry", I would be intentionally implying that I am not really going to school right now . Likewise, the statement "If John discovers that Mary is in New York, he will get angry" can be used to imply that Mary is actually in New York.
I think the key word here is "discovers", specifically present tense and not past or future tense. That essentially implies that Mary is in fact—right now, presently—in New York. As I summarized in a comment below, for this work problem in a basic textbook, I think it's appropriate that we infer the least implications, i.e., not include an in the future implication as a possible interpretation of the original premise. I think if the book authors wanted to write "in the future" they probably would have, and it would have made the answer clearcut (i.e., there would be no entailments). But the fact that they left it out suggests either they wanted the opposite (an entailment such that A entails B), or they wanted everyone to be confused. As pernicious as some school textbook authors may seem to be, I think it's reasonable we can assume they did not intend the latter... :P
Thus, I believe that fairest answer is that (A) presupposes (B).