Comments in K2 follow a "flat" style. If you're logged in in your site's frontend, you can respond to comments directly. You can't respond to comments in the Joomla backend (in K2).

This is what I'm saying. There is no "answer" function. You just add a new comment. If you wish to make it more obvious, use @name or @username so the person you reference understands who your comment is for.