Is the word Man's a correct plural possessive form?
If by 'Man' you mean to imply 'all of mankind', a collective noun:* yes, man's can be considered a plural possessive. e.g. 'Man's inhumanity to man.' But if you mean several men, i.e the plural of man, an adult hiuman male, then* no, the correct possessive form would be men's. e.g. 'Where is the men's room?'