assign validation as required field

Here you go, I just wrote it myself:

add_filter( 'registration_errors', 'select_user_role_error', 10, 3 );
function select_user_role_error( $errors, $sanitized_user_login, $user_email ) {
  if(isset($_REQUEST['wp_rar_user_role']) && $_REQUEST['wp_rar_user_role'] == 'contributor') :

    $errors->add( 'policy_error', "<strong>ERROR</strong>: Please select an option." );
    return $errors;
  endif;
  return $errors;
}

Just change contributor to whatever your role is and should check if you have selected the right one.