Fill in the blank with the correct conditional form: If he ___ harder, he would have passed.

This question tests your knowledge of the third conditional, which is used to talk about hypothetical situations in the past that did not happen and their hypothetical results. The structure for the third conditional is: If + past perfect (had + past participle), subject + would have + past participle.

The second part of the sentence, "he would have passed," clearly indicates the result clause of a third conditional. Therefore, the conditional clause must use the past perfect tense.

• C: had studied is the correct answer because "had studied" is the past perfect form, fitting the required structure "If he had studied harder." This implies he did not study harder, and consequently, he did not pass.
• A: studies is incorrect. This is the present simple, used for the first conditional (e.g., "If he studies, he will pass"), which refers to future possibilities.
• B: studied is incorrect. This is the past simple, used for the second conditional (e.g., "If he studied, he would pass"), which refers to hypothetical present or future situations.
• D: studying is incorrect. This is a present participle or gerund and cannot form a complete verb phrase for a conditional clause on its own.