Flags - show a flag marker even if user cannot modify it
You could add a new boolean field Answer to comments which has a default value of 0. Next, create a Rule that changes the boolean value of a flagged comment to 1. The rest is theming; use the following code in your comment.tpl.php file:

&lt;?php if ($comment-&gt;field_answer['und'][0]['value']): ?&gt;
  &lt;mark&gt;This is the answer&lt;/mark&gt;
&lt;?php endif; ?&gt;