An adjectival clause always includes a/an

A. conjunction.

B. direct object.

C. relative pronoun.

D. infinitive verb.

?

The answer is C. "relative pronoun."

First, it will contain a subject and verb. Next, it will begin with a relative pronoun [who, whom, whose, that, or which] or a relative adverb [when, where, or why]. Finally, it will function as an adjective, answering the questions What kind? How many? or Which one?

So the answer is C. relative pronoun.