It is because you forgot to pass in event into the click function:
$('.menuOption').on('click', function (e) { // <-- the "e" for event
e.preventDefault(); // now it'll work
var categories = $(this).attr('rel');
$('.pages').hide();
$(categories).fadeIn();
});
On a side note, e is more commonly used as opposed to the word event since Event is a global variable in most browsers.