Example How to Show only first level options in select field in drupal 8
if ( $form['#id'] == 'views-exposed-form-documents-page-1'){
$options_act = $form['category']['#options'];
foreach ($options_act as $key => $activity) {
if (isset($activity) && !is_object($activity)) {
if (strpos($activity, '-', 0) === 0) {
unset($options_act[$key]);
}
}
}
$form['category']['#options'] = $options_act;
}
