इस and ये have the meaning, yes? I chose ये and it got accepted. But I can also say इस गाड़ियाँ लाल हैं or is that only for singular?
I'm pretty sure it's only for singular.
Also I think इस is the oblique case (e.g. इस घर में... or इस आदमी की बहऩ...)
What is oblique case
Oblique case is the way you have to change everything when there is a post position.