The dance squad had eight members when it won the competition.
A relative clause can be used as an adjective or an adverb. Relative clauses must contain both a subject and a verb. They also begin with a relative pronoun (question words and that). The relative clause in the last option is "when it won the competition". It starts with "when" which is a relative pronoun. It describes when the action takes place which makes the entire clause an adverb. It has a subject "it" and a verb "won". Options B and C should be immediately thrown out because they do not have a relative pronoun. Option A does not work because there is no subject after "that".