Topic: Conditional Sentence,

Best Answer Explained in Below-:

The sentence "If he - a humane being, he would not have done this" calls for a conditional structure that reflects a hypothetical or unreal situation in the past. In English grammar, the third conditional is used for situations that did not occur in the past and speculates about their outcomes. The structure of third conditional sentences generally involves using "if" followed by the past perfect tense (if + past perfect) and the main clause with "would have" plus the past participle (would have + past participle).

In this context, the correct option is C) "had been," which forms the past perfect tense. The use of "had been" indicates that the condition (being a humane being) did not happen at any time before the action in question (the action he performed). The sentence essentially expresses that if he had been a humane being, he would have acted differently.

The other options (A) "is," (B) "was," and (D) "have been" do not appropriately reflect the hypothetical nature of the statement regarding the past. "Is" implies present tense, "was" indicates a specific past moment, and "have been" signifies a continuing experience into the present. Thus, "had been" is the only option that accurately conveys the intended meaning of a hypothetical past scenario.