ANSWER: A. Adverb
An adverb clause is defined exactly as it is in the question. It is used in a sentence to tell where, when, how, or to what extent something was done. It is similar to adverbs themselves, since adverbs also modify an adjective, verb, or another adverb based on time, place, degree, manner, etc.
A main clause is a clause that can stand alone on its own while an adjective clause will function as an adjective itself which answers either "how many?", "what kind?" or "which one?".
