Answer:
By the time they arrived
Step-by-step explanation:
A clause is a group of related words with at least a subject and a verb that can be either independent or subordinate. We can identify which type of clause a group of words with those characteristics is because while a subordinate clause does not express a complete thought on its own, an independent clause does express a complete idea on its own and can stand alone as a full sentence.
From all options given, only option C is a subordinate clause because it does not express a complete thought on its own, and in fact, it should be part of a bigger sentence in order to make sense.