In which sentence does the underlined adverb modify an adjective? Young owls grow slowly. Young owls usually grow slowly. Very young owls grow slowly. Young owls grow quite slowly.

The correct answer is Very young owls grow slowly.

Here, the adverb is very and the adjective is young, so the adverb is used to modify the adjective, or to show a particular degree of the adjective (how young? very young.).

In the rest of the options, there is only one adverb slowly, and it's not modifying an adjective, but rather the verb.