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;
}
?>