Choice "C" is correct. Generally, under the mailbox rule, acceptance is effective when sent. However, an offeror may opt out of the mailbox rule by stating that the acceptance must be received by a certain date to be effective. Fine's offer here required receipt by April 4. Moss' acceptance was received after the April 4th deadline.
Choice "a" is incorrect. The offer did not require any particular method of acceptance, only receipt within a specified time. Thus, any reasonable means of acceptance was authorized.
Choice "b" is incorrect. Since the offeror required that the acceptance be received to be effective, the mailbox rule would not be applicable.
Choice "d" is incorrect. The offer stated that an acceptance had to be received by April 4 and the attempted acceptance here was received on April 5. Thus, it was ineffective because the offer had already terminated on the date attempted acceptance was received.