Answer: C) An adjective.
Explanation: a verb is a word that expresses an action made by the subject of the sentence, an adverb is a word that modifies a verb, an adjective or another adverb, an adjective is a word that modifies a noun, and a noun is a word that names a place, person, or object. In this case the word that best completes the sentence is an adjective, because it will modify the noun "boulders" making complete sense with the rest of the sentence.
