We're aware there is currently a slight technical issue with the quizzes when accessed on mobile.
Fortunately there is a workaround for making the quizzes work on mobile! The best thing to do is enable desktop mode on your mobile device browser.
To do so, click on the menu button at the top right of the browser and enable the option Request Desktop Site.
Please give that a try and let us know how you get on.
Alternatively, you could complete the quiz on a computer. It should work well there.