Answer: C) After they rested a few days, the aliens revived sufficiently to explore.
Step-by-step explanation: an independent clause is a phrase that has a subject and a predicate and that has complete meaning on its own. A dependent or subordinate clause is a phrase that doesn't have complete meaning on its own. When we have a subordinate clause followed by an independent clause, we must separate them with a comma, so the correct answer is option C, we have the subordinate clause "after they rested a few days" and the independent clause "the aliens revived sufficiently to explore" separated by a comma.