The correct answer is D. knows.
The verb “know” is a regular verb, which means that the past tense and past participle are formed by adding -ed to the present tense form. However, the present tense form of “know” is irregular, and it is not formed by adding -ed. The present tense form of “know” is “knows.”
In this sentence, “He speaks as the one who knows,” the verb “knows” is in the present tense because it is describing a habitual or repeated action. The speaker is saying that the person he is talking about always speaks as if he knows everything.
The other options are incorrect because they are not the present tense form of the verb “know.” Option A, “knowing,” is the present participle form of the verb. Option B, “knew,” is the past tense form of the verb. Option C, “is knowing,” is the present progressive form of the verb.
I hope this explanation is helpful. Please let me know if you have any other questions.