Five days’ vacation is the correct way to rewrite the phrase "a vacation of five days." Option C is correct.
In the words ends in S you do not have to add another s or es. You just should add the (') for the possesive,
Possesives may be used to talk about time and duration, as in this case.
Furthermore, it is also used to show that something belongs to someone or something.
According to the rule, we use ’s after a singular noun and ’ after a plural noun.