There are two mistakes in you code
1st : keycode of c is 67
2nd : text variable is undefined.
So the final code would be
$(document).keydown(function(e){
if(e.ctrlKey && e.altKey && e.which == 67){
e.preventDefault();
prompt("Copy to clipboard: Ctrl+C, Enter","text");
}
});
you can check the fiddle here