i added new option in menu options because i needed css class in <a> tag, searched for a while and didn't find any example so had to do the following.
$NavOptions = array(
'wrapper_list_type' => 'ul', // ul, ol, nav, div, etc.
'list_type' => 'li', // li, a, span, etc.
'menu_css_id' => 'navbarSupportedContent', // a CSS ID for the menu
'menu_css_class' => 'navbar-nav ml-auto', // a CSS Class for the menu
'current_class' => 'active',
'default_title' => 0, // 0=show saved titles;1=show actual/current titles
'default_class' => 'nav-item', // a CSS class to apply to all menu items
'ahref_class'=>'nav-link'
);
added following line in MarkupMenuBuilder.module
$this->set('linkClass', isset($options['ahref_class']) ? $options['ahref_class'] : '');
// if $iTag is empty, apply css id and classes to <a> instead
if(!$iTag) $out .= "\n\t<a{$itemCSSID}{$class}{$newtab} href='{$m->url}'>{$m->title}</a>";
else $out .= "\n\t<{$iTag}{$itemCSSID}{$class}>\n\t\t<a{$newtab} href='{$m->url}' class='{$o->linkClass}'>{$m->title}</a>";
suggest me if there is any better way out!!