Fill in the blank: He behaves as if he ______ the boss.

The sentence "He behaves as if he ______ the boss" requires the subjunctive mood because it describes a hypothetical or unreal situation. When "as if" introduces a condition that is contrary to fact or highly improbable, the past subjunctive form of "to be" is used, which is "were" for all subjects, including singular ones like "he."

• "He behaves as if he is the boss" would imply he might actually be the boss, or it's a real possibility.
• "He behaves as if he was the boss" is grammatically incorrect in this context for a hypothetical situation.
• "He behaves as if he were the boss" correctly conveys that he is not the boss, but he acts like it.
• "He behaves as if he be the boss" is incorrect; "be" is used in other subjunctive contexts, but not typically after "as if" for a hypothetical condition.

Therefore, the correct option is C.

The final answer is $\boxed{\text{C) were}}$.