the answer is b. here is why here are some examples
An adverb clause is a group of words that function as an adverb in a sentence. The clause can modify or describe verbs, adverbs, and adjectives. In general, adverb clauses add information that elaborates on when, where, why, how, how much or under what condition the action in the sentence takes place.
An adverb clause isn't just any group of words, however. A clause must contain a subject and a verb to be complete. An adverb clause also begins with a subordinating conjunction, such as "after," "if," "because" and "although." If you see a group of words in a sentence that acts like an adverb but does not have both a subject and a verb, it's an adverb phrase.
