The correct answer to this question would depend on who it is asked to. I must admit I have never thought about my favorite letter. After seeing the options to choose from, for me, the correct answer to the question would be A, A. I chose this because my daughter's name begins with A and she is my favorite person in the world.
I also chose it because it starts the alphabet. Whenever one sings the alphabet song, they must start with A. In itself, A is an important letter and out of all the answers, it is my favorite.