noun modifiying a noun

[absal]

what is correct and why ? plz
1- This bathroom is for ladies only. It is a lady room.
2- This bathroom is for ladies only. It is a lady's room.
3- This bathroom is for ladies only. It is a ladies' room
4- This bathroom is for ladies only. It is a ladies room.

[Quizmonster]

"It is a ladies' room" is the only correct version, because the room is for use by ladies - plural - and so the apostrophe after the plural 's' shows it is theirs.

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[Tweed]

It's a question of possession. To check where the apostraphe should go, just ask yourself who owns the object in question. So, e.g. The ball is Peter's. Who owns the ball? Peter. In the above, who owns the room? Ladies.

[Carlos26]

I'd say it was lady's room, otherwise the the second sentence is irrelevant.  In this case wouldn't it be implying social class (and as we'd be implimg class of each lady who goes in, it would be in singular)rather than use for females only,  as this is already staetd in sentence one.