In my opinion, the answer should be option (C) instead of option (A). The statement above is a question, however, no wh- question words (including how) is used. Thus, it is a yes, or no question. Additionally, factoring in the meaning of the statement, it can be understood that this question is meant to reconfirm a fact. Obviously, to reconfirm the fact, one needs to discuss the fact. The article used in case where the object is known to the speaker is the determinate article, or 'the'. Thus, instead of 'a', the answer should be 'the', or option (C).