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?

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

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.