Final answer:
An adverbial clause modifies a verb, adjective, or another adverb within a sentence. It does not modify nouns, pronouns, or prepositions. Therefore, the adverbial clause can modify an adverb. Option B
Step-by-step explanation:
An adverbial clause is a dependent clause that functions as an adverb, meaning that it modifies a verb, an adjective, or an adverb in the sentence. Adverbial clauses can indicate time, place, condition, contrast, and cause or effect. Let's look at an example sentence where an adverbial clause is modifying a verb:
Because she practiced daily, she became a fantastic piano player.
In this case, the adverbial clause 'Because she practiced daily' provides a reason for the action of the verb 'became'. Hence the adverbial clause modifies verbs, adjectives, and other adverbs but not nouns, pronouns, or prepositions. If we refer to your choices, the correct answer would be option B) Adverb.