Use this CodeLet to remove "N/A" option from radio button list options.
Place this CodeLet to your custom module file(.module) or Create a Custom module for Drupal 8 as per documentation https://www.drupal.org/docs/8/creating-custom-modules
CodeLet
<?php/** * Implementation of hook_element_info_alter * Remove N/A option from radio button field * @param array $types */function MYMODULE_element_info_alter(array &$types) { if (isset($types['radios'])) { $types['radios']['#process'][] = '_MYMODULE_remove_radio_na'; }}/** * remove_radio_na * A Callback function for MYMODULE_element_info_alter * @param array $element * @return modified $element */function _MYMODULE_remove_radio_na($element) { if (!empty($element['#field_name']) && $element['#field_name'] == 'field_range') { unset($element['_none']); } return $element;}?>