O.K., since the response begins with Non, this signals that you're going to need a verb in the negative, so that will eliminate the choice les ont parlés which is affirmative. Also, the object of the contracted à in the question is the third person plural noun phrase les clients, so the first choice, ne m'ont pas parlé, could not be correct since the only pronoun here is m' which of course is the contracted first person singular pronoun me.
That leaves ne nous ont pas parlé and ne leur ont pas parlé, and you can now actually pick the correct answer without fully understanding the complicated rules of French indirect and object pronoun order in affirmative and negative sentences (but if you'd like to punish yourself with a lesson in these, just ask me). The choice will be the second, ne leur ont pas parlé, since leur is the third person plural indirect object pronoun corresponding to English to them, the "them" here being 'the clients'.
Compare this with ne nous ont pas parlé which has nous, the first person plural indirect (and object) pronoun which means 'to us' or sometimes just 'us'. The point is there's no mention of nous is the question, so using any form of nous in the response would make little sense contextually or pragmatically.