"Is this your car?"
There is nothing in the English to indicate whether or not “your" is plural. Both 你的 and 你们的 should be accepted.
I think “车” rather than “汽车” is also correct here. I think in every day speech referring to cars as 车 is okay. What do you guys think?
As others have commented, this can be either the singular "you" or the plural. So the singular you should not be marked incorrect.