The correct option is B b
Let’s examine the sentence: “Neither of them are going to attend the party on 10th October.” The sentence states a fact about each one of them. It uses singular pronoun, ‘neither.’ Hence, the sentence requires a verb used for singular nouns according to the rules of subject-verb agreement. So, ‘are’ should be substituted by ‘is’ in the sentence. Hence, the error is in part b of the sentence. So, Option B is the correct answer.