The correct option is B Knows
Let’s examine the sentence: “He is knowing the secret.” The sentence states the fact of his knowing the secret as a forever truth and not as an activity happening and continuing in the present. So, the correct tense for the sentence should be simple present tense and not in present continuous tense as used in its present construction. So, the present continuous tense of the sentence should be changed to simple present tense to improve it. Therefore, the improved version of the sentences is, “He knows the secret.” Hence, Option A is the correct answer.