Please be aware that the answers to the questions are embedded on the page. There's no server side checking. There is also no IP validation or whatsoever. It's just a interactivity widget to make your visitors more involded with your website's content.

Can I change the A), B), C), etc?
Only via a so called file override. You need to copy the blocks/page_quiz/view.php file and put it under /application/blocks/page_quiz/view.php. Then you can modify HTML output to your needs.

Can I have multiple correct answers?
Yes, you can.

How can I enable the Bootstrap theme?
First add the block as you'd normally do. Then click on the block and select "Design & Custom Template". Then press the gear icon and select Bootstrap as "Custom Template".