I doubt if I ________ resisted the temptation

The correct answer is "could have" because the sentence talks about whether it was possible to resist temptation in the past. "Would have" is used for imaginary situations, but it is not correct here. "Should have" means it was the right thing to do, but that’s not what the sentence is saying. "Will have" talks about the future, so it doesn’t fit. The best choice is "could have" because it expresses possibility in the past. The correct sentence is: "I doubt if I could have resisted the temptation."

Option C is the correct answer.