An adjectival clause always includes a/an

a. conjunction.
b. direct object.
c. relative pronoun.
d. infinitive verb.
?

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.