Fill in the blank: What would you have done if you _____ your driving test this time?

The sentence requires a specific form of the verb to indicate a hypothetical situation in the past. The phrase "What would you have done if you _____ your driving test this time?" implies an unreal condition that did not happen. In this context, the correct form is the past perfect, which is "hadn't passed."

Using "hadn't passed" indicates that the driving test is in the past and the outcome is contrary to the actual result; it suggests that there was an opportunity to pass the test that did not occur. This structure effectively communicates the hypothetical nature of the situation: the speaker is asking about what actions would have been taken had the outcome been different.

Other options do not fit correctly into the sentence's context regarding time and conditionality. For example, "didn't pass" is simple past and does not convey the necessary hypothetical meaning. "Wouldn't have passed" is a mixed conditional, implying a future possibility rather than a past situation. "Haven't passed" indicates a present perfect tense that refers to a current situation, which is not applicable when discussing an event that has already taken place. Therefore, "hadn't passed" is the most suitable choice.