Step 1: Meaning of 'Question'.
A 'question' means an inquiry or something asked to gain information.
Step 2: Match the synonym.
The word 'inquiry' directly means seeking information, hence it is the synonym of 'question.'
Step 3: Eliminate wrong options.
(A) Letter – written message, not synonym.
(C) Envy – jealousy, unrelated.
(D) Keep – to hold, unrelated.