Product Enquiry form for Variable WooCommerce Products - https://www.damiencarbery.com/2018/04/product-enquiry-form-for-variable-woocommerce-products/

ninja-forms-options-to-select-field.php

<?php
/*
Plugin Name: Ninja Forms - Set options in select field.
Plugin URI: https://www.damiencarbery.com/2018/04/product-enquiry-form-for-variable-woocommerce-products/
Description: Set the options in a specific select field in Ninja Forms.
Author: Damien Carbery
Version: 0.1
*/


add_filter( 'ninja_forms_render_options', function($options,$settings){
	if ( 'select_product_attributes' == $settings[ 'key' ] ) {
		$options = array();
		$options[] = array(
          'label' => "A label",
          'value' => 'a'
        );
		$options[] = array(
          'label' => "B label",
          'value' => 'b'
        );
		$options[] = array(
          'label' => "C label",
          'value' => 'c'
        );
	}

   return $options;
},10,2);