“& did not start a character reference. (& probably should have been escaped as &.)” w3c markup

Did you include the semi-colon after &amp? Your link should be

<a href="https://stackoverflow.com/questions/15970738/index.php?option=com_content&amp;view=article&amp;id=6&amp;Itemid=4">

Note that if you paste this url into the address bar of your browser it won’t work – the browser converts the & into & when you click the link in the page.