The house is large ___ is quite old-fashioned.

The sentence "The house is large ___ is quite old-fashioned" requires a conjunction to connect two contrasting ideas. The house being large is generally a positive attribute, while being old-fashioned can be seen as a negative or contrasting one. Option D, "But," is the correct coordinating conjunction to use here because it introduces a contrast or something contrary to what has just been stated. It effectively highlights the difference between the house's size and its style.

• Option A, "And," is used to add information or connect similar ideas, not contrasting ones.
• Option B, "Or," is used to present an alternative or choice.
• Option C, "Which," is a relative pronoun used to introduce a dependent clause, and its usage here would be grammatically incorrect without a comma and a different sentence structure (e.g., "The house, which is large, is quite old-fashioned.").

The contrast between "large" and "old-fashioned" necessitates the use of "but" for proper grammatical flow and meaning.