Whatever he says, I will not __________ him.

The correct answer is A: believe. This sentence features the modal auxiliary verb "will." In English grammar, modal verbs (such as will, would, can, could, may, might, must, shall, should) are always followed by the base form of the main verb, also known as the infinitive without "to." The base form is the simplest form of the verb, without any endings like -s, -ed, or -ing. In this sentence, "will not" expresses a future negative intention. Therefore, the verb that follows "will not" must be in its base form. "Believe" is the base form of the verb. Options B ("believed") is the past tense or past participle, C ("believing") is the present participle, and D ("believes") is the third-person singular present tense. None of these forms are correct after a modal verb. The structure "will not believe" correctly conveys the speaker's firm decision not to trust or accept what the other person says, regardless of their statements.