Add class to current category menu item

You can use is_category() You can check the documentation here

function show_parent_categories( $args = array() ) {

$args = array(  
    'hierarchical' => 1,
    'show_option_none' => '',
    'hide_empty' => 0,
    'parent' => $category->term_id,
    'taxonomy' => 'product_cat'
);

$product_categories = get_terms( 'product_cat', $args );

$count = count($product_categories);
 if ( $count > 0 ){
 echo "<div id='main-categories-wrapper'><ul id='main-categories'>";
 foreach ( $product_categories as $product_category ) {
   echo '<li><a class="'.(is_category($product_category->slug)?'active':'').'" href="' . get_term_link( $product_category ) . '">' . $product_category->name . '</li>';

 }
 echo "</ul></div>";
}
}