Which sentence is punctuated correctly?
a. the aliens suffered from exhaustion, after traveling several light years.
b. they needed some relaxation, when they finally landed their spaceship.
c. after they rested a few days, the aliens revived sufficiently to explore.
d. they needed all their energy, once they started hiking the rough terrain.

Your answer would be C.

Explanation:

C is the sentence punctuated correctly. It starts with an introductory prepositional phrase which requieres a comma because it has more than four words. Intruductory phrases are like clauses except that they do not have a subject and a verb, they rely on the subject and the verb of the main clause. They set the stage for the main part of the sentence. The general rule is that when the introductory phrase has more than four words, you should use a comma after it.

i used to know but i learned it a long time ago sorry

explanation: