We could have gone to the beach if the weather ____ better.

• A. Would be
• B. Is
• C. Had been
• D. Will be

Answer: (C)

You're dealing with a past unreal condition: a situation in the past that didn’t actually happen. For that kind of sentence, the if-clause uses the past perfect, and the result clause uses could have + past participle. Here, “If the weather had been better” describes a past situation that didn’t occur, and “we could have gone to the beach” expresses what was possible in that past. The other options don’t fit time: would be, is, and will be refer to present or future time, not a past hypothetical. So the best choice is the past perfect form.