The woman in the car, ---- was very beautiful, is my landlady.
A) who B) which
Although I agree with lanuageguy that the sentence is not good, there is no doubt at all about the meaning. It is the car that is beautiful, not the lady.
"---- was very beautiful" is isolated by commas.
It is a descriptive relative clause
So it must be placed immediately following the noun it describes.
That noun is car.
Car is inanimate.
The choice is between who and which.
It must be which.
The woman in the car, which was very beautiful, is my landlady.
This is quite silly, so the "rules of English" or the structure of the sentence must be silly.
This time it is the "rules of English" that are silly. It is quite clear that "woman in the car" is a syntactic unit and it is human. The descriptive relative clause refers to this syntactic unit, and therefore who is appropriate.
The woman in the car, who was very beautiful, is my landlady.
So far, so good.
But aren't the tenses all askew?
She was beautiful, but she is my landlady.
I suppose it's possible that I know she was once beautiful but isn't now.