Correct Answer:
B. would have
The correct phrase to complete the sentence "Had he worked hard, he __________ passed the exam" is "would have." This sentence is an example of the third conditional, which is used to talk about hypothetical situations in the past that did not happen and their hypothetical results.
The standard structure for the third conditional is:
- If + past perfect (for the condition)
- would have + past participle (for the result)
In this specific sentence, the "if" is omitted, and an inversion is used: "Had he worked hard" is equivalent to "If he had worked hard." Following this structure, the result clause requires "would have" followed by the past participle of the main verb ("passed").
Therefore, the complete and grammatically correct sentence is: Had he worked hard, he would have passed the exam.