The correct option is B (b)
Let’s examine the sentence: “I have found the man whom I know is the thief.” The sentence uses “whom” wrongly to indicate the “man” preceding it. Instead of “whom,” the relative pronoun “who” should be used here. The sentence has three clauses: “I have found the man,” “I know,” and “whom is the thief.” In the clause, “whom is the thief,” “whom” cannot be used as the subject of the verb “is” as it is an objective pronoun and so it should be replaced by the subjective pronoun “who.” We use ‘who’ to talk about the subject of a sentence, and we use ‘whom’ to talk about the object of the verb. Hence, the correct version of the sentence would be, “I have found the man who I know is the thief.” So, the error is in Part b of the sentence. Hence, Option B is the correct answer.