Answer :
The sentence that is punctuated correctly is, After they rested a few days, the aliens revived sufficiently to explore. This sentence is a complex sentence consisting of an independent and dependent clause. When the dependent clause comes first, a comma should be used to introduce the independent clause. The rest of the given sentences above need not to be separated with a comma.
It would be C, After they rested a few days, the aliens revived sufficiently to explore.