Which sentence contains an adjective clause?
a. the chanterelle, a mushroom, is often yellow.
b. whose pizza has mushrooms?
c. mushrooms, which come in many varieties, can be expensive.
d. edible mushrooms are a great source of potassium.

c. Mushrooms, which come in many varieties, can be expensive.

Adjective clauses modify the noun or the pronoun in the sentence's main clause. The first thing to do is to identify the two clauses in the sentence.

First clause: Those may enter the park (the main clause)
Second clause: whose tickets have been punched (the subordinate clause)

Since adjective clauses generally start with a relative pronoun, it is clear that the second clause is the adjective clause. The relative pronoun is "which". Another clue is that adjective clauses are always the subordinate clause. It modifies the pronoun those.