Quick and dirty prototype for cross-off functionality in Moodle quizzes. Plenty of limitations to this and lots of suboptimal coding practices. But good enough for a quick demo and "something like this?" questions.

gistfile1.js

var answers = document.querySelectorAll('.multichoice .answer div');

for (var i=0, l=answers.length; i<l; i++) {
        answers[i].appendChild(document.createTextNode('\u2717'));
        answers[i].addEventListener('click', function( e ) {
                if (e.target===this) {
                        if (this.getAttribute('style') === 'text-decoration: line-through') {
                                this.setAttribute('style', ''); 
                        } else {
                                this.setAttribute('style','text-decoration: line-through');
                        }
                }
        }); 
}