I am sure I would have regretted it if I ________ to take the job.
had had
had decided
had refused
or past perfect tense of some other verb
To complete the sentence, you would need to fill in the blank with the correct verb form. In this case, the verb should be "had decided" or "had chosen" to maintain parallelism with the verb "would have regretted." The complete sentence would then be:
"I am sure I would have regretted it if I had decided/ chosen to take the job."
To determine the correct verb form, it is important to consider the tense and the structure of the sentence. In this case, "would have regretted" indicates a hypothetical outcome in the past, so the past perfect tense is used to express that the decision was made before the regret would have occurred.