Which word in the sentence is the predicate adjective? The Rocky Mountains must have looked impossibly high to the weary explorers. A. Rocky B. high C. weary D. impossibly

The predicate adjective is something that describes the subject of the sentence, such as

subject is predicate adjective.

in our sentence it's "impossibly high" (full phrase) or, more specifically "high" (answer B):

Rocky Mountains (are) high (paraphrasing the sentence)