How can I make an option only appear for one or a few countries? And, how can I do the opposite of that, so every country gets the option, but specific countries don't?

For events options which you want to show to a specific nation: TAG = ?? where ?? is the nation(s) you want.

For example, UK and USA:

TAG = ENG
TAG = USA

And to exclude nation(s), you wrap it in NOT:

NOT = {
TAG = ENG
TAG = USA
}