Read the dictionary entry.
re • frain [ri-freyn]
1. (verb) to resist doing something
2. (noun) the chorus of a musical piece
which sentence uses refrain as a noun?
the flight attendant requested that the passengers refrain from smoking.
students should refrain from using their cell phones during the lecture.
experienced audience members refrain from clapping until the end.
the haunting refrain stayed with the audience long after the show ended.

D) The haunting refrain stayed with the audience long after the show ended.

Explanation: the denotation of a word is its literal meaning, the definition that we can find in a dictionary, a word can have several denotations (or definitions). In the given dictionary entry we can see two different denotations of the word "refrain" (one is a verb and the other is a noun). From the given options, the only sentence that uses the word "refrain" as a noun is the corresponding to option D, in the other ones, it is used as a verb.