Which of the folwing is corre3ct? a) this is more expensive than any car in this showroom. b) this car nis most expensive than any other car in this showroom. c) this car is most expensive in this showroom. d) this car is more expensive in this showroom.

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. :)