The correct answer is A.
B is incorrect because instead of "more expensive than," it says "most expensive than," which doesn't make any sense.
C is incorrect because it implies the car is more expensive in this showroom than anywhere else it could be, which with the context of the answers, is completely irrelevant.
D is incorrect because it either serves as a fragment of a sentence which is incorrect, or fails by saying "in this showroom" instead of making a comparison, "than this showroom."
Therefore, A is the only sentence with proper grammar that makes sense. :)
