The most probable answer is option 2: Rainsford killed Zaroff and slept in the general's bed. This is clear to me because of what is stated in the passage that he had never slept in a better bed, meaning he won the fight for it.
Option 1 is not probable because it wasn't a matter of letting the other person sleep in the bed, but to fight for the right to do so.
Option 3 does not represent the outcome of the story because it's Rainsford the one who wins the fight and gets to sleep on the bed.
Option 4 "None of these" isn't the right option because it doesn't include the story's denouement in itself.