According to some histoirians, if Napolian had not invaded Russia, he----- the rest of Europe. ?

The correct answer to the question is C) "would have conquered" because it uses the conditional perfect tense, which is appropriate for discussing hypothetical situations in the past.

In this sentence, we are speculating about an alternative historical outcome: if Napoleon had not invaded Russia, there is an implied consequence of him conquering the rest of Europe. This consequence relies on a hypothetical scenario that did not actually happen. The structure "if [past perfect], would have [past participle]" effectively conveys that the event (Napoleon conquering Europe) was dependent on the unfulfilled condition (not invading Russia).

Option A) "had conquered" implies a completed action in the past but fails to indicate the speculative nature of the scenario. Option B) "would conquer" suggests a future outcome based on a present condition, which does not fit the context of discussing a past event. Finally, option D) "conquered" is in the simple past tense and does not convey the conditionality required by the sentence. Thus, C) "would have conquered" is the only choice that accurately reflects the speculative nature of the statement regarding historical events.