In each of the following questions, choose the option that best completes the gap(s). If I had worked hard, I _______ very high marks in the exam.

This sentence uses the third conditional (also called the past unreal conditional), which describes a hypothetical situation in the past and its imagined result. The structure is: If + past perfect, would have + past participle.

The if-clause here is "If I had worked hard" (past perfect: had + past participle worked). This tells us the speaker did not actually work hard. The main clause must therefore express the unreal result using would have + past participle.

The past participle of "score" is scored. The correct completion is therefore would have scored, giving: "If I had worked hard, I would have scored very high marks in the exam."

The other options fail for specific grammatical reasons:

• would have score pairs "would have" with the bare infinitive instead of the past participle; after "would have," English requires the past participle form (scored).
• will score is the simple future tense and belongs in first-conditional sentences about real future possibilities, not past hypotheticals.
• can score expresses present ability or possibility and is incompatible with a past-perfect if-clause.

When you see an if-clause in the past perfect tense, always pair it with would have + past participle in the main clause.