functions.php

<?php
/**
 * This hook will make question description field optional.
 *
 * @param array $form Form arguments.
 * @return array
 */
function my_ap_make_question_description_optional( $form ) {
  $validate = $form['fields']['post_content']['validate'];
  
  //remove required from validation.
  if ( is_array( $validate ) ) {
    if ( ( $key = array_search( 'required', $validate ) ) !== false ) {
      unset( $validate[ $key ] );
    }
  } else {
    $validate = str_replace( 'required', '', $validate );
  }
  
  // Update existing value.
  $form['fields']['post_content']['validate'] = $validate;
  
  return $form;
}

add_filter( 'ap_question_form_fields', 'my_ap_make_question_description_optional' );