Final answer:
The boys' bathroom is very messy.
Step-by-step explanation:
The boys' bathroom is very messy.
In this context, 'boys'' is incorrect because it indicates possession with a double apostrophe, which is not the correct usage. The correct form is 'boys' with just one apostrophe after the 's.'
Grammar rules require using the apostrophe after the 's' to indicate possession. Since the bathroom belongs to the boys, the correct form is 'boys' bathroom.'
The correct answer to the question "The ____ bathroom is very messy" is C. boys'. This is because the term "boys" is plural and the possessive form is indicating that the bathroom belongs to more than one boy. Therefore, the apostrophe is placed after the plural noun "boys" to show possession by the group.
Therefore the correct option is A. boy's.