Mond relates the best argument for the brave new world society. what is it?
I see that you have tagged this with "Chapter 16" so I will use lines from that chapter, even though I think that his arguments in Chapter 17 are at least as persuasive as those in Ch. 16. Basically, Mond's best argument in Ch. 16 is that the brave new world keeps people safe.
In the world that John and Helmholtz want to have, people can have strong emotions. They can enjoy beautiful things. They can really, truly, be human. Mond agrees that there are benefits to this. However, he points out that these things come at a price. Here is a good line:
What's the point of truth or beauty or knowledge when the anthrax bombs are popping all around you?
What Mond is saying is that it is better to be safe, to be alive, than to have the excitement and intense life that you could have in our "regular" world. The civilization has traded emotions and such for safety and that is Mond's argument.