Read the paragraph.The next day, we began the trek up Mount Magnificent. At first, the climb was an easy walk on a well-marked trail. But soon, we were scrambling over _______ boulders and scaling walls of stone.

The missing word describes the boulders: it tells what kind of boulders the hikers scrambled over. Words that describe nouns are adjectives, so the correct choice is “an adjective.”

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