Translation:Are you a woman?
If onna by itself means woman, why have the の人 with it if means the same?
I'm not sure about the details, but from what I've gathered from other comments, just 女 is what it'd say if you need to for example give your gender in a form you're filling out, and sounds a bit rude in normal conversation. 女の人 is more polite.
Onna no ko is girl but Onna no hito is woman. You'd need to add "no hito" or "no ko" if you need to distinguish between the two. Or, it would be necessary if you needed to be more formal as has been mentioned.
In my very limited experience with Japanese, I've noticed that additional politeness is often added. O is added before the words for bath or tea. San, sensei, or sama are added to names. I've started keeping track of context and formality.
This question immediately makes me think of Hakata Tonkotsu Ramens, where it is a legit question.