C. adjective clause
As you may know, a noun is a person, place, or thing. An adverb describes a verb. And, an adjective describes a noun. When nouns, adverbs, and adjectives are more than just one word, they are called clauses. The bolded words “whom Mr. Stein chose” are actually describing the noun “student.” And, because we know that words describing nouns are adjectives or adjective clauses, it is understood that “whom Mr. Stein chose” is an adjective clause.