Pre-select WooCommerce first variation option. Copy code and put in your child theme's function.php or anywhere you see fit. Use at your risk, enjoy! I mean, what's life without taking risks? 🚀

pekky-wc-file.php

<?php
/**
 * Pre-selects first variation option.
 *
 * @param array  $args
 * return array
 */
function pekky_wc_select_first_variation_option( $args ) {
	$args['selected'] = $args['options'][0];
    return $args;
}
add_filter( 'woocommerce_dropdown_variation_attribute_options_args', 'filter_dropdown_variation_args', 10, 1 );