Punctuations are used in a sentence to organize and structure it properly. Moreover, it is used to create clarity and sense or to stress in a sentence. For example quotation marks " ", full stop (.), commas and so on.
Commas are frequently used punctuation in a sentence, hence they are used to create a short pause in a sentence.
In the above sentences, the sentence which is punctuated correctly is C. After they rested a few days, the aliens revived sufficiently to explore.
When starting with the dependent clause, the sentence is punctuated after the dependent clause. In the above sentence dependent clause is “After they rested a few days”, hence as compared to other sentences it is punctuated correctly.