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 through terrain.

The question is asking us to say which sentence is punctuated correctly among the following sentences. For this, we can us the SOV rule: if something interrupts the Subject-Verb-Object -(other) order in a sentence, is should be separated by commas. here, it's option C: C) After they rested a few days (adverbial phrase), the aliens (S) revived (V) sufficiently to explore. So there must be a comma after "days" , but such a comma is not necessary in other cases. 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 through terrain."