Change ‘Choose an Option’ variation dropdown label in WooCommerce. Code goes in function.php file of your active child theme (or active theme).

functions-singular_name.php

/* OR you can use singular_name instead of name like: */

add_filter( 'woocommerce_dropdown_variation_attribute_options_args', 'filter_dropdown_variation_args', 10 );
function filter_dropdown_variation_args( $args ) {
    $args['show_option_none'] = apply_filters( 'the_title', get_taxonomy( $args['attribute'] )->labels->singular_name );

    return $args;
}

functions.php

/* You can use wc_attribute_label() dedicated function to get the product attribute label name. */

add_filter( 'woocommerce_dropdown_variation_attribute_options_args', 'filter_dropdown_variation_args', 10 );
function filter_dropdown_variation_args( $args ) {
    $args['show_option_none'] = apply_filters( 'the_title', wc_attribute_label( $args['attribute'] ) );

    return $args;
}