This wordplay riddle uses “letter” in a different sense from the alphabet. Instead of thinking about A, B or C, think about something that can physically contain a letter.
Answer: A mailbox.
A mailbox can have one letter inside it, but the mailbox itself is certainly not part of the alphabet. That double meaning is the entire trick.
It is a classic riddle because the wording pushes you toward letters as symbols, while the answer depends on remembering that a letter can also mean a piece of mail.
