WordPress Ajax Data problem

    $.post(ajaxurl,{ 
        action:'master_ajaxurl',                
        option: $(this).find("option:selected").val(), // use option also here
       },
       function(data){
         //adds the echoed response to our container
        // alert(data);
        $("#details").html(data);
      }
   );

Move action and functions into function.php file

make some changes as commented

add_action('wp_ajax_master_ajaxurl', 'master_ajaxurl');
add_action('wp_ajax_nopriv_master_ajaxurl', 'master_ajaxurl');

    function master_ajaxurl() {

    $myOpt = $_POST['option']; // use option instead of data
    echo bringDetails($myOpt);
    die();
    }                     

    
    **page.php**    
    
    $(document).ready(function () {
        var ajaxurl="----admin_url----";
        $('#combobox').click(function(){

            $.post(ajaxurl,{                
                action:'master_ajaxurl',
                option: data to send, // key => value
                                option2: 'hello'
            },
                function(data){
                //adds the echoed response to our container
                alert(data);
                //$("#details").html(data);
            }
        );
        });
    });


    **functions.php**
    function master_ajaxurl() {

    $myOpt = $_POST['option'];
    echo bringDetails($myOpt);
    die();
     } 

must use echo and die() in your ajax output function.
this is working for me. Hope this works now.