The correct answer is Very young owls grow slowly. Here, the adverb is <em>very </em>and the adjective is <em>young, </em>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 <em>slowly, </em>and it's not modifying an adjective, but rather the verb.
