Which word is a singular noun ? a) athletic b) men c) clowns d)dancers

I believe you have a typo when it comes to the first option - it should be athletics, not athletic. The word athletic is an adjective, and the word athletics is a noun.
That being said, the correct answer is A. athletics. Even though this noun ends in -s, it is still singular. The rest of the options are all plural nouns. 

Ithink it’s c or a