Topic: Conditional Sentence,

Best Answer Explained in Below-:

The phrase "He-to see us if he had been able to." is structured to express a conditional scenario that did not happen in the past. The use of "had been able to" implies a hypothetical situation in which the subject (he) lacked the ability to see us. In English, this type of conditional often employs the "would have" construction to describe the outcome of an unreal past situation.

Option D, "would have come," correctly completes the conditional sentence. It indicates that if he had possessed the ability (i.e., if the conditions were different in the past), he would have taken the action of coming to see us. This construction effectively links the inability in the past to the prospective action.

In contrast, the other options do not suit the context of the sentence. For example, "had come" (A) denotes a completed action in the past, "came" (B) refers to a simple past action without conditional context, and "is coming" (C) describes a present or future event, which does not align with the retroactive condition stated. Therefore, the appropriate choice is D) "would have come," as it accurately reflects the conditional nature of the statement.