The correct answer is C. Young owls grow quite slowly
Step-by-step explanation:
An adverb is a word that acts as a modified of verbs, adjectives or even other adverbs, indeed adverbs describe the frequency, degree, place, time, etc of verbs (actions) or adjectives (words that describe nouns). This occurs in the case of "Very young owls grow slowly" as in this sentences the underlined adverb "very" that means extremely and modifies the adjective "young", although there is also and the second adverb "slowly" that modifies the verb "grows". According to this, the sentence with an underlined adverb that modifies an adjective is "Young owls grow quite slowly".