Type/Paste in the text area below that reads "Replace the content in this area ..." Then underline the words/answers you want to blank-out. (see the example)

There are two approaches:

You can copy and paste from any document editor or html wysiwyg editor. Make underlines or other style changes in your editor, then copy and paste into the text area. It's been tested with LibreOffice Writer in both document and web mode, and in the Blogger editor compose mode (for Google Blogger pages).

Also:

You can use the built-in simple style editor.
Highlight the text you wish to style and then click a style button (e.g. bold, underline, xxl etc.)
Use ctl + z for undo and ctl + y for redo. However, once you click the style editor, you can only undo back to that point and not before. The solution is to just copy and cut, then repaste with ctl + shift + v (paste unformatted). Everything returns to plain text and you retype as desired.