The correct option is C (c)
Let’s examine the sentence: “You will come to my party tomorrow, isn't it?” The sentence uses the question tag, “isn’t it?” Question tags are the short questions that we put on the end of sentences – particularly in spoken English. If the main part of the sentence is positive, the question tag is negative and if the main part of the sentence is negative, the question tag is positive. The question tag uses the same modal verb and pronouns as present in the main part of the sentence. Now, if we see the sentence, the question tag is not using the modal verb used in the main part of the sentence. The question tag will use negative form of modal verb, “will,” which is “won’t.” Also, the question tag will use the pronoun “you,” and not “it,” as “you” is present in the main part of the sentence. Hence, the correct sentence would be, “You will come to my party tomorrow, won’t you?” As the error is in part c of the sentence, Option C is the correct answer.