In the sentence

Question

"Yellowstone National Park is one of the largest national parks in the U.S."

is the prepositional "of the largest national parks" modifying "one"? If so, is the prep phrase being used as an adjective or adverb. I say "adverb" because "is" is a linking verb, therefore "one" is a predicate noun", thus "of the largest national parks" is modifying an adjective, which makes it an adverb prep phrase, My book says the answer is adjective - why?

Answer 1

This phrase is an adjective phrase because (as you said) it modifies "one." In this sentence "one" is a predicate pronoun, thus its modifiers are adjectives.