How can I use country state picklist enabled for org in my apex class?

Go to Setup --> Data Management --> State and Country Picklists

Follow the steps :
But make sure it will change your organization data,

In order to enable this option you must to do the following three options to enable.

1. Configure states and countries. --> You need to configure which country you want.

2. Scan now. --> This will scan all your existing data in contact.

3. Convert your data. --> Then convert all your existing data.

Once you finish those steps you will able to enable this feat