Students who purchase parking spots are able to park in the lot closest to the entrance.

Is it a, restrictive phrase, restrictive clause, nonrestricive phrase, or nonrestrictive clause?

A restrictive clause is an adjective clause that adds essential information to a sentence. This is unlike a nonrestrictive clause, the omission of which would not change the meaning of the sentence. Because "who purchase parking spots" is an essential part of the sentence, the clause is a restrictive clause.

The answer is ... restrictive clause just took the pre test.