An adverb clause will meet three requirements:
First, it will contain a subject and verb.You will also find a subordinate conjunction that keeps the clause from expressing a complete thought.Finally, you will notice that the clause answers one of these four adverb questions: How? When? Where? or Why?Read these examples:
Tommy scrubbed the bathroom tile until his arms ached.How did Tommy scrub? Until his arms ached, an adverb clause.I personally think the best answer is C.