add_filter('body_class', 'hide_div_if_post_belongs_to_category');
function hide_div_if_post_belongs_to_category($classes) {
        if ( is_singular('post') )
        {
            if (in_category($category_id , get_the_ID()))
            {
                $classes[] = 'hide-this-div';
            }
        }
        return $classes;
}