The event would have been successful if you _________ able to come.
- ((a))
Are
- ((b))
Had been
- ((c))
Have been
- ((d))
Would have been
Show Answer
Had been
The sentence is in the perfect continuous tense which can be noticed because of the usage of "been".
When you see "would have" in a sentence it means that the action didn't actually happen, because something else didn't happen first.
From the sentence we know that the "event" has already happened.
Hence the correct answer is "had been" as it would convey the correct tense, which is the past perfect continuous.
Option 2 is correct.