Let’s examine the sentence: “(a) Though his is good (b) he is (c) mischievous.” The verb ‘is’ requires a subjective pronoun as its subject. A subjective pronoun acts as the subject of a sentence—it performs the action of the verb. The subjective pronouns are he, I, it, she, they, we, and you. So, ‘his’ should be replaced by ‘he’ in the sentence. Hence, the error is in Part A of the sentence. So, Option A is the correct answer.