I am thinking of one of the following five letters
You can ask me one question to guess which letter it is.
No question related to the number of the letter in the alphabet order, like
" If the letters A to Z go from 1 to 26, what is the number of the letter you are thinking?" Or anything that relates to that order
No compound questions that may contain "and" "or". No multiple questions disguised as single questions.
No obvious questions like "what letter are you thinking or what words begin with your letter etc"
I am reasonably knowledgeable but am allowed to ask for clarification of your question or check it to be sure