Which of the Following Is the BEST Reason to Choose

Which of the following is the BEST reason to choose a memo format for a full formal report's "letter of transmittal"?

A) Memos are usually shorter than letters; therefore, it's less work for you to write a memo.
B) Memos seem more casual; because the report is formal, this is your only opportunity to appear friendly and laid back.
C) The full formal report was written for an internal audience; internal audiences expect the memo format.
D) It really doesn't matter; memos and letters are interchangeable and you can use either format.
E) Actually, you should always use a letter format.