Solution broke after attempting to optimize the code.

var questions = [
    ['What year was Marc born?', '1992'],
    ['What day was Marc born?', '9'],
    ['What month was Marc born?', '2']
];
var rightAnswers = 0;
var wrongAnswerCount = 0;
var wrongAnswerList = [];
var rightAnswerList = [];
var listHTML;

function buildList(arr) {
    var listHTML = '<ul>';
    for (var i = 0; i < arr.length; i += 1) {
        listHTML += '<li>' + arr[i] + '</li>';
    }
    listHTML += '</ul>';
    return listHTML;
}

function ask() {
    for (var questionCount = 0; questionCount <= (questions.length - 1); questionCount += 1) {
        var answer = prompt(questions[questionCount][0]);
        console.log(questionCount);
        if (answer === questions[questionCount][1]) {
            rightAnswers += 1;
            rightAnswerList.push(questions[questionCount][0]);
        } else {
            wrongAnswerCount += 1;
            wrongAnswerList.push(questions[questionCount][0]);
        }
    }
    document.write('<p> Great! You got ' + rightAnswers + ' correct!' + '</p>');
    document.write('<p> Here are the ones you got right:</p>');
    buildList(rightAnswerList);
    document.write(listHTML);


    if (wrongAnswerCount > 0) {
        document.write('<p>You got ' + wrongAnswerCount + ' wrong too, these ones:');
        buildList(wrongAnswerList);
        document.write(listHTML);
    }
}
ask();