How to Add Security Line input into Contact Form - php

Question

I have that PHP, jQuery, and AJAX Powered Contact Form Bootstrap
It use Form Validator.
I want to add Security Question Input into that form (like here), to prove the user is human.
Not something complicated. Question like that:
Question: What is the capital city of New York?
Answer: New York
If the user write "new york" and submit, the form will send.
Here is the code:
Here is the index.html HTML Markup code:
<form role="form" id="contactForm" class="contact-form" data-toggle="validator" class="shake">
<div class="form-group">
<div class="controls">
<input type="text" id="name" class="form-control" placeholder="Name" required data-error="Please enter your name">
<div class="help-block with-errors"></div>
</div>
</div>
<div class="form-group">
<div class="controls">
<input type="email" class="email form-control" id="email" placeholder="Email" required data-error="Please enter your email">
<div class="help-block with-errors"></div>
</div>
</div>
<div class="form-group">
<div class="controls">
<input type="text" id="msg_subject" class="form-control" placeholder="Subject" required data-error="Please enter your message subject">
<div class="help-block with-errors"></div>
</div>
</div>
<div class="form-group">
<div class="controls">
<textarea id="message" rows="7" placeholder="Massage" class="form-control" required data-error="Write your message"></textarea>
<div class="help-block with-errors"></div>
</div>
</div>
<button type="submit" id="submit" class="btn btn-effect"><i class="fa fa-check"></i> Send Message</button>
<div id="msgSubmit" class="h3 text-center hidden"></div>
<div class="clearfix"></div>
</form>
Here is the contact-form-script.js code:
$("#contactForm").validator().on("submit", function (event) {
if (event.isDefaultPrevented()) {
// handle the invalid form...
formError();
submitMSG(false, "Did you fill in the form properly?");
} else {
// everything looks good!
event.preventDefault();
submitForm();
}
});
function submitForm(){
// Initiate Variables With Form Content
var name = $("#name").val();
var email = $("#email").val();
var msg_subject = $("#msg_subject").val();
var message = $("#message").val();
$.ajax({
type: "POST",
url: "assets/php/form-process.php",
data: "name=" + name + "&email=" + email + "&msg_subject=" + msg_subject + "&message=" + message,
success : function(text){
if (text == "success"){
formSuccess();
} else {
formError();
submitMSG(false,text);
}
}
});
}
function formSuccess(){
$("#contactForm")[0].reset();
submitMSG(true, "Message Submitted!")
}
function formError(){
$("#contactForm").removeClass().addClass('shake animated').one('webkitAnimationEnd mozAnimationEnd MSAnimationEnd oanimationend animationend', function(){
$(this).removeClass();
});
}
function submitMSG(valid, msg){
if(valid){
var msgClasses = "h3 text-center tada animated text-success";
} else {
var msgClasses = "h3 text-center text-danger";
}
$("#msgSubmit").removeClass().addClass(msgClasses).text(msg);
}
Here's the form-process.php
<?php
$errorMSG = "";
// NAME
if (empty($_POST["name"])) {
$errorMSG = "Name is required ";
} else {
$name = $_POST["name"];
}
// EMAIL
if (empty($_POST["email"])) {
$errorMSG .= "Email is required ";
} else {
$email = $_POST["email"];
}
// MSG SUBJECT
if (empty($_POST["msg_subject"])) {
$errorMSG .= "Subject is required ";
} else {
$msg_subject = $_POST["msg_subject"];
}
// MESSAGE
if (empty($_POST["message"])) {
$errorMSG .= "Message is required ";
} else {
$message = $_POST["message"];
}
$EmailTo = "your#email.com";
$Subject = "New Message";
// prepare email body text
$Body = "";
$Body .= "Name: ";
$Body .= $name;
$Body .= "\n";
$Body .= "Email: ";
$Body .= $email;
$Body .= "\n";
$Body .= "Subject: ";
$Body .= $msg_subject;
$Body .= "\n";
$Body .= "Message: ";
$Body .= $message;
$Body .= "\n";
// send email
$success = mail($EmailTo, $Subject, $Body, "From:".$email);
// redirect to success page
if ($success && $errorMSG == ""){
echo "success";
}else{
if($errorMSG == ""){
echo "Something went wrong :(";
} else {
echo $errorMSG;
}
}
?>
What do i need to add in the php file (or in the js file as well) so i can prove the user is human?