The word that would best elevate the sentence to a formal tone is “increase”.
The noun “bump”, among many of its meanings, is used to mean an increase in something, to become bigger.
The noun “increase” is used to indicate a situation in which the number or amount of something is bigger.
Actually, the correct choice would be A. "....the battle's only survivor/swam up and away from those silent corpses...".
I just took the whole unit test, C. Was wrong.
It's A.
The answer that you are looking for is C