Answer: a. the guys' skateboards.
Option A is the one that correctly uses the possessive form. Most of the time, when we are turning a noun into a possessive, we add an "s" if the noun is singular:
Ex. the bike of the man = the man's bike
However, when we are dealing with plurals, the apostrophe is added to indicate the possessive, but no "s" is needed anymore:
Ex. the skateboard of the guys = the guys' skateboard