Question

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js"></script>
<script  type="text/javascript">
$(document).ready(function(){
   $('#btnSubmit').bind('click', function(){
        var option = $('#option1').val();
        if(option == ''){
            alert('Answer empty');
        }
        return false;
   }); 
});
</script>

…

<input type="radio" id="option1" name="option1 />
<input type="submit" name="btnSubmit" id="btnSubmit" value="submit" />

…

=> error click button submit not alert

Answer 1

you should modify you code a bit get the value of :checked radio button and if its not checked it means the value is empty and alert the answer is empty

for example

$('#btnSubmit').bind('click', function(){
    alert("");
    var option = $('#option1:checked').val();
        if(!option){
            alert('Answer empty');
        }
        return false;
   });  

here is the working example http://jsfiddle.net/L8VY8/