WooCommerce customizing Variation Dropdown

Try this in your theme/functions.php

add_filter( 'woocommerce_variation_option_name', 'customizing_variations_terms_name', 10, 1 );
function customizing_variations_terms_name( $term_name ){

if(is_admin())
    return $term_name;

global $product;
$second_loop_stoped = false;

// Get available product variations
$product_variations = $product->get_available_variations();

// Iterating through each available product variation
foreach($product_variations as $variation){

    $variation_id = $variation['variation_id'];
    $variation_obj = new WC_Product_Variation( $variation_id );

    ## WOOCOMMERCE RETRO COMPATIBILITY ##
    if ( version_compare( WC_VERSION, '3.0', '<' ) ) # BEFORE Version 3 (older)
    {
        $stock_status = $variation_obj->stock_status;
        $stock_qty = intval($variation_obj->stock);

        // The attributes WC slug key and slug value for this variation
        $attributes_arr = $variation_obj->get_variation_attributes();
    }
    else # For newest verions: 3.0+ (and Up)
    {
        $stock_status = $variation_obj->get_stock_status();
        $stock_qty = $variation_obj->get_stock_quantity();

        // The attributes taxonomy key and slug value for this variation
        $attributes_arr = $variation_obj->get_attributes();
    }

    if(count($attributes_arr) != 1) // Works only for 1 attribute set in the product
        return $term_name;

    // Get the terms for this attribute
    foreach( $attributes_arr as $attr_key => $term_slug){
        // Get the attribute taxonomy
        $term_key = str_replace('attribute_', '', $attr_key );

        // get the corresponding term object
        $term_obj = get_term_by( 'slug', $term_slug, $term_key );
        if( $term_obj->name == $term_name ){ // If the term name matches we stop the loops
            $second_loop_stoped = true;
            break;
        }
    }
    if($second_loop_stoped)
        break;
}
if( $stock_qty>0 )
    return $term_name .= ' - ' . $stock_status . ' ('.$stock_qty.')';
else
    return $term_name .= ' - ' . $stock_status;

}

Posted here thanks to loictheaztec :

Show stock status next to each attribute value in WooCommerce variable products