WordPress select field – show current value?

By referencing to the source code, this function supports an argument called value which you can pass one of the keys inside the options array like this:

function woo_add_spin() {
    woocommerce_wp_select([
        'id'       => '_select_nf',
        'label'    => __( 'Select #spins: ', 'woocommerce' ),
        'selected' => true,
        'value'    => '24',
        'options' => [
            '24' => __( '24', 'woocommerce' ),
            '36' => __( '36', 'woocommerce' ),
            '48' => __( '48', 'woocommerce' )
        ]
    ]);
}

By looking inside the source code, WooCommerce also tries to fetch the value of the select field using the id field and the post meta but, you can’t take advantage of this built-in feature because you’re saving the settings inside the wp_options table.