Concept:The verb "stop" is followed by a gerund (verb+ing) to mean cease an action, not by an infinitive (to + verb).Explanation:In part a), "stop to make noise" implies pausing one action in order to start another.But the intended meaning is to cease making noise.Therefore, the correct form is "stop making noise" — using the gerund.The phrase "stop to make" would mean "pause (something else) in order to make noise", which is incorrect here.So, error is in part a).Answer:Option A (a)