Making the country_id drop-down field as a required field?

Your best bet is to use Validation:

https://laravel.com/docs/5.4/validation#quick-writing-the-validation-logic

Basically in the controller method that receives the form data, you will define what fields are required. If validation fails, the response will be a redirect back to the url from which it received the post, along with an error bag.

https://laravel.com/docs/5.4/validation#quick-displaying-the-validation-errors

required works, if your default selected option has value="", so if they don't choose a country it will submit an empty string, which will fail the required rule.

<option value="" selected>Choose your country</option>
...
<option value="usa">United States</option>
<option value="can">Canada</option>
...
etc

@Cronix thank you, in my controller I have the following:

    public function getRegister(CountryRepository $countryRepository)
    {
        
        //$countries = $countryRepository->lists();
        $countries = [0 => 'Select a Country'] + $countryRepository->lists()->toArray();
        return view('auth.register', compact('socialProviders', 'countries'));
    }