Question

Which answer is the sentence fragment?

Answer 1

- - D.Steamy humid days in the summer.

This is because there is no meaning of the sentence, nor would it make any sense to say it to someone. 

As of all the others make sense and are not considered "fragments"

A fragment is something that is unfinished, not completed. 

Answer 2

D. Steamy humid days in the summer. There is no verb in this fragment.