Answer:
C. The Glastonbury Festival, which is held on every June, is the largest music festival in the world.
Step-by-step explanation:
A relative clause is a clause that modifies a noun or pronoun in the main clause of a sentence. In this sentence, the relative clause is modifying the noun "festival," and it is providing additional information about the festival. The relative pronoun "which" is used to introduce the relative clause, and it is placed immediately after the noun being modified. The clause is then followed by a verb, in this case "is held," which provides additional information about the festival.