Unreal past conditional sentences, also known as third conditional sentences, describe hypothetical situations in the past that did not happen and their hypothetical results. The standard structure for these sentences is "If + past perfect, would have + past participle."
Option B, "Had he studied, he would have passed," is correct. This sentence uses an inversion of the third conditional structure, where "If" is omitted and "Had" is placed at the beginning of the clause. This is a grammatically correct and common way to express an unreal past condition, equivalent to "If he had studied, he would have passed."
Option A is incorrect because "If he would have studied" is grammatically incorrect for the conditional clause; "would have" is used in the result clause. Option C, "If he studied, he would have passed," is incorrect because "If he studied" uses the simple past, which is for unreal present or future conditions (second conditional), not unreal past. Option D is incorrect because "he passes" is in the simple present, which does not fit the past conditional structure.