Answer and Explanation:
John asks Mond why they have to have whole groups of identical Deltas. Why not make everyone an Alpha Plus like Bernard and Helmholtz?
When asked that question, Mustapha Mond replies by pointing out that chaos would be the result of a society made up entirely of Alpha Pluses. According to him, in such a society, no one would want to do the menial jobs that Epsilons and Deltas performed. He even mentions that an Alpha Plus society was once attempted on Cyprus, only to end up in civil war.
Are you satisfied with Mond’s answer?
Yes. The society described in "Brave New World" is an unfair one, and I would much rather prefer a different society - one in which how intelligent we are and what jobs we perform are not chosen for us. But, since this is the society they have in the story, I believe his answer is a rational one, especially if we take into consideration that they have made an experiment with a society made of Alpha Pluses.