grammatical number - How should I pluralize letters?

Sometimes, it becomes necessary to refer to, for instance, the instances of the letter "A" in a document. When doing so, how should I format the plural letter? I have seen A's, "A"s, "A"'s, and "A"es. Is one of these the optimal formatting, or is it something else entirely?

I would like to ignore the option of rephrasing the sentence, if at all possible.