The correct answer is C.

Explanation: The sentence "He behaves as if he a ____________ governor." requires the use of the subjunctive mood, which is used to express hypothetical or non-real situations. In English, the subjunctive mood for the verb "to be" is "were" for all subjects, including singular subjects.

- C. were is correct because it correctly uses the subjunctive mood to indicate that the behavior is hypothetical or unreal, suggesting that he acts as if he were a governor, though he may not actually be one.

Other options:
- A. is: This is indicative and would not be appropriate for expressing a hypothetical situation.
- B. was: This is past tense and does not fit with the hypothetical nature of the sentence.
- D. are: This is present plural and does not match the singular subject "he."

Thus, "were" is the correct choice to complete the sentence.