The question tests the understanding of English vocabulary related to real estate. The correct answer is 'B. Steep' because in the context of real estate, 'steep' is often used metaphorically to describe a very high price. Option A is incorrect because 'mountain' refers to a natural landform. Option C is incorrect because 'expensive' is a general term for high cost but is not specific to real estate prices. Option D is incorrect because 'costly' is also a general term for something that is expensive but does not specifically relate to the metaphor used in real estate.