You need to create an advanced theme as explained on this page. Depending on exactly where you want the content to go, you'll need to override the a_item_main or a_item_content functions. An example would be:
public function a_item_main($a_item)
parent::a_item_main($a_item); // outputs the answer as normal
$this->output('<div>Your custom HTML here</div>');
Do note that Adsense limits you to 3 ads per page so you can't go and put ads after every answer if there are 10.
If the above is too complicated you can try the Widget Anywhere plugin which will allow you to put HTML code after the question (before the answers) and at the bottom of the page. But it doesn't do after every answer.