You can set restrict the country field within an address question so that only one country is selected. To enable this setting, you can enable "Restrict addresses to a country". Here's how:

1. Navigate to the form editor.
2. Click on the address field you'd like to add a default country to.
3. Switch the "Restrict addresses to a country" toggle to on.
4. A dropdown selection will appear. Select the country you wish to use from the list provided.

Address Field Configuration - Restrict Addresses to a Country