Change "choose an option" in WooCommerce.

functions.php

<?php 

//Customise the dropdown 'choose an option'
add_filter('woocommerce_dropdown_variation_attribute_options_args', 'wcvdc_dropdown_choice', 10);


// Displays the custom "Choose an option" on the front end
function custom_dropdown_choice( $args ){
  global $product;

  $args['show_option_none'] = "Select a: " . strtolower(wc_attribute_label($args['attribute'],$product));

  return $args;
}