You have to look at the event's target.
With your HTML:
<div class="a">
<div class="b">Hello</div>
<div class="c">World</div>
</div>
You can use this Javascript (jQuery):
$('.a').on('click', function(event){
var $target = $(event.target);
if($target.is('.a')){
alert('You actually clicked .a');
} else {
alert('You did not click .a');
}
});
Here's a quick demo with jQuery.