SLameN,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
.red{
background-color: Red;
}
</style>
</head>
<body>
<div class='test-data'>
<div class='question' data-id='1' id='question-1'>
<p class='q'>Вопрос 1</p>
<p class='a'><input type='radio' id='answer-1' name='question-1' value='1'>
<label for='answer-1'>Ответ 1</label></p>
<p class='a'><input type='radio' id='answer-2' name='question-1' value='2'>
<label for='answer-2'>Ответ 2</label></p>
<p class='a'><input type='radio' id='answer-3' name='question-1' value='3'>
<label for='answer-3'>Ответ 3</label></p>
</div>
<div class='question' data-id='2' id='question-2'>
<p class='q'>Вопрос 2</p>
<p class='a'><input type='radio' id='answer-4' name='question-2' value='4'>
<label for='answer-4'>Ответ 1</label></p>
<p class='a'><input type='radio' id='answer-5' name='question-2' value='5' >
<label for='answer-5'>Ответ 2</label></p>
</div>
</div>
<div class="pagination">
<a class="nav-active" href="#question-1">1</a>
<a href="#question-2">2</a>
<a href="#question-5">3</a>
</div>
<button class='center btn' id='btn'>Результаты</button>
<script>
btn.onclick = function() {
var a = document.querySelectorAll('.pagination a');
for (var i=0; i<a.length; i++) {
var sel = a[i].getAttribute('href') + ' input:checked';
if(document.querySelector(sel)) a[i].classList.remove('red');
else a[i].classList.add('red');
}
}
</script>
</body>
</html>