Had we arrived at the restaurant earlier, we _________________ order a full meal.

The correct answer is "would have been able to," which fits seamlessly in the conditional structure of the sentence. The original clause "Had we arrived at the restaurant earlier" establishes a hypothetical situation in the past. In English, when talking about unreal or contrary-to-fact conditions in the past, the third conditional structure is commonly used, which consists of "if" + past perfect in the "if" clause and "would have" + past participle in the main clause.

Using "would have been able to" indicates that, under the condition of arriving earlier, there was a definite possibility of ordering a full meal. It conveys certainty about the outcome that could have happened in that imagined circumstance.

The other options do not convey the same level of certainty or fit as neatly into the third conditional structure. "Could have ordered to," for example, incorrectly constructs the verb "could" with "ordered to," which is grammatically off. "Might have been able to" introduces a sense of uncertainty or possibility rather than the stronger notion of capability implied in the original context. Finally, "should have been able to" suggests an obligation or expectation that aligns less with the hypothetical nature of the scenario at hand. Thus, "would have been able