You would be looking to use the 'multichoice' scripting command. In Emerald it displays a message with multiple answers that you can choose between with the number of the answer chosen stored in variable 0x800D which can then be compared. It would look something like this:
Code:
...
multichoice 0x0 0x0 0x2 0x1 // Put's multichoice 0x2 in the top left corner of the screen, the B button cannot be used to exit
copyvar 0x8000 0x800D // Moving the player's answer from variable 0x800D to 0x8000 as 0x800D will be overwritten automatically by later commands
compare 0x8000 0x0 // Checking if the value in 0x8000 is 0x0
if 0x1 goto @ThunderStone // If it is, they chose the first answer and get a Thunder Stone
compare 0x8000 0x1 // Checking if the value in 0x8000 is 0x1
if 0x1 goto @FireStone // If it is, they chose the second answer and get a Fire Stone
compare 0x8000 0x2 // Checking if the value in 0x8000 is 0x2
if 0x1 goto @LeafStone // If it is, they chose the third answer and get a Leaf Stone
...
This is a very basic write up of how the multichoice system works but I'd suggest reading diegoisawesome's XSE tutorial to properly learn its functions. In addition, if you're using Emerald's binaries rather than the decomps, you can use
LSA's multichoice box editor to change the number of available answers/ total boxes.