The correct answer is “were”.
The sentence “He behaves as if he were a king” is in the past subjunctive mood. The past subjunctive is used to express unreal or hypothetical situations. In this case, the speaker is saying that the man behaves in a way that is characteristic of a king, but he is not actually a king.
The other options are incorrect because they are not in the past subjunctive mood. “Was” is in the simple past tense, “has” is in the present perfect tense, and “had” is in the past perfect tense. These tenses are not appropriate for expressing unreal or hypothetical situations.
Here are some more examples of the past subjunctive:
- I wish I were rich.
- If I were you, I would go home.
- I would have gone if I had known.
In these sentences, the speaker is expressing a wish, a condition, or a hypothetical situation. The past subjunctive is used to make these statements more tentative or less certain.