woocommerce_dropdown_variation_attribute_options_html.php

<?php
/**
 * Hook: woocommerce_dropdown_variation_attribute_options_html.
 * Modify the HTML for the variation options.
 */
function plugin_republic_dropdown_variation_attribute_options_html( $html, $args ) {
  // Modify the $html here
  return $html;
}
add_filter( 'woocommerce_dropdown_variation_attribute_options_html', 'plugin_republic_dropdown_variation_attribute_options_html', 10, 2 );