<div class="demo-container">
<form action="" method="post" id="demoForm">
<h4>1. Which country has the most earthquakes?</h4>
<div class="questions-container">

<div class="form-check">
<input type="radio" class="form-check-input" name="question-01" id="question-01-A" value="A" required /><label for="question-01-A">Japan</label>
</div>

<div class="form-check">
<input type="radio" class="form-check-input" name="question-01" id="question-01-B" value="B" /> <label for="question-01-B">Australia</label></div>

<div class="form-check">
<input type="radio" class="form-check-input" name="question-01" id="question-01-C" value="C" /> <label for="question-01-C">USA</label></div>

<div class="form-check"> <input type="radio" class="form-check-input" name="question-01" id="question-01-D" value="D" />
<label for="question-01-D">Italy</label></div>

</div>

<h4>2. Which country has the most islands?</h4>
<div class="questions-container">

<div class="form-check">
<input type="radio" class="form-check-input" name="question-02" id="question-02-A" value="A" required /><label for="question-02-A">USA</label></div>

<div class="form-check">
<input type="radio" class="form-check-input" name="question-02" id="question-02-B" value="B" /> <label for="question-02-B">Sweden</label></div>

<div class="form-check">
<input type="radio" class="form-check-input" name="question-02" id="question-02-C" value="C" /> <label for="question-02-C">Albania</label></div>

<div class="form-check">
<input type="radio" class="form-check-input" name="question-02" id="question-02-D" value="D" /> <label for="question-02-D">Germany</label></div>

</div>

<h4>3. Which country has the most lawyers per capita?</h4>

<div class="questions-container">

<div class="form-check">
<input type="radio" class="form-check-input" name="question-03" id="question-03-A" value="A" required /><label for="question-03-A">Belgium</label></div>

<div class="form-check">
<input type="radio" class="form-check-input" name="question-03" id="question-03-B" value="B" /> <label for="question-03-B">North Korea</label></div>

<div class="form-check">
<input type="radio" class="form-check-input" name="question-03" id="question-03-C" value="C" /> <label for="question-03-C">South Korea</label></div>

<div class="form-check">
<input type="radio" class="form-check-input" name="question-03" id="question-03-D" value="D" /> <label for="question-03-D">USA</label></div>

</div>

<h4>4. Which country has the most fresh water?</h4>

<div class="questions-container">
<div class="form-check"> 
<input type="radio" class="form-check-input" name="question-04" id="question-04-A" value="A" required /><label for="question-04-A">Italy</label></div> 

<div class="form-check">
<input type="radio" class="form-check-input" name="question-04" id="question-04-B" value="B" /> <label for="question-04-B">The Netherlands</label></div>

<div class="form-check">
<input type="radio" class="form-check-input" name="question-04" id="question-04-C" value="C" /> <label for="question-04-C">USA</label></div>

<div class="form-check">
<input type="radio" class="form-check-input" name="question-04" id="question-04-D" value="D" /> <label for="question-04-D">Brazil</label></div>

</div>

<div class="message-container">

<div id="message"></div>
</div>
<div class="submit-button-container">
<input type="submit" value="Submit" class="btn btn-secondary submit-button" />
</div>

</form> 
</div>

.demo-container {
  margin: 4em auto;
  max-width: 600px;
}

.demo-container h4 {
  font-size: 1em;
  border-bottom: 1px dashed #ececec;
}

.questions-container  {
  margin-bottom: 2em;
}

.submit-button-container  {
  margin-top: 2em;
  text-align: right;
}

.submit-button {
  width: 160px;
  text-align: center;
}

.message-container {
  background-color: rgba(255, 118, 0, 0.20);
  border-radius: 4px;
  text-align: center;
  margin: 1em 0;
  padding: 4px 20px;
}

<script>
$(function () {

$("#message").html("Answer these questions");
$('#demoForm').on('submit', function (event) {

$.ajax({
type: 'post',
url: 'post.php',
data: $('#demoForm').serialize(),
success: function (data) {
$("#message").html(data);
}

});
event.preventDefault();
});

});
</script>

<?php
$answer_01 = $_POST['question-01'];
$answer_02 = $_POST['question-02'];
$answer_03 = $_POST['question-03'];
$answer_04 = $_POST['question-04'];

$totalRight = 0;

if ($answer_01 == "A") { $totalRight++; }
if ($answer_02 == "B") { $totalRight++; }
if ($answer_03 == "C") { $totalRight++; }
if ($answer_04 == "D") { $totalRight++; }

// result:

if ($totalRight == 0 ) {
$result = "<strong>All</strong> answers are <strong>incorrent</strong>";
}

if ($totalRight == 1 ) {
$result = "Only <strong>one</strong> answer is <strong>correct</strong>";
}

if ($totalRight == 2 ) {
$result = "<strong>Two</strong> answers are <strong>correct</strong>";
}

if ($totalRight == 3 ) {
$result = "<strong>Three</strong> answers are <strong>correct</strong>";
}

if ($totalRight == 4 ) {
$result = "<strong>All</strong> answers are <strong>correct</strong>";
}

echo "" . $result . "";
?>