The correct option is A (a)
Let’s examine the sentence: “Unless you stop to make noise at once, I will have no option but to bring the matter to the attention of the police.” The verb ‘stop’ is acting as a transitive verb in the sentence. A transitive verb requires an object to complete its sense. Hence, the verb “stop” would be followed by the object and not the preposition “to.” This object can be provided by a noun, pronoun or gerund. Hence, the sentence needs the gerund of the verb “make,” i.e. “making.” So, the preposition “to” should not exist between “stop” and “making.” The correct form of the sentence would be: “Unless you stop making noise at once, I will have no option but to bring the matter to the attention of the police.” Hence, the error is in Part A of the sentence. So, Option A is the correct answer.