[Script] Where to create or change multiple choice options?

In short I want to create a multiple choice option when talking to Steven Stone where he offers a Thunderstone, Firestone, or Waterstone.
Example so you know exactly what I mean by multiple choice.

I know how to work with pre-existing multi choice options and to call different labels/functions, but I don't know how to change the actual options that appear.

I have found in include/constants/script_menu.h a list of defined multiple choice variables, I've added "MULTI_STONES" to the list, but I have no idea where the option names are set.

I appreciate any help.

Assuming you're working with pokeemerald, this should be of some use for you.

Ah awesome, thanks. Don't know how I managed to miss that going through the simple modifications directory.