The correct answer is letter C. From the given choices, letter indicated the revision level of a revised drawing. In engineering, whenever a change occurs in a revised drawing, revision letters should follow these guidelines: letter should be in sequence and must be in upper case starting with letter A and omitting letter I, O, Q, X, and Z because these numbers typically appear like numbers and may confuse others.
Once all the letters have been exhausted, the letter which should follow Y should be AA through AY. If ever AA to AY have been exhausted, the next sequence should be BA to BY and so on and so forth. However, please note that revision letters should and must not exceed two characters.