The correct option is D Third Conditional
The Third Conditional is used to refer to imaginary past actions, where the conditions for the event were NOT satisfied, and the result of this unsatisfied condition. Put simply, it talks about a past that didn't happen and speculates on what would happen if it did. The structure for the third conditional is: if + past perfect tense, would have + verb form (can be conjugated). In this case, the verb is 'to be' and thus the sentence adheres to the structure of the Third Conditional - i.e, Option D).