pass the dropdown select value in to controller

<select id="discount"  >

            <option value="" ></option>
            <option value="0"></option>
            <option value="1"></option>
            <option value="2"></option>
        </select>

function discountPostback() {
           debugger;
           var res = document.getElementById('discount').value;
          // __doPostBack('discount', res);
           $.ajax({
               type: "POST",
               url:'@Url.Action("GetUserActivity")',
               data: { id: res },
               success: function (data) {
                   debugger;
               }
           });

       }

public ActionResult GetUserActivity(short? type)
     {
         return Json(_reportRepository.GetUserActivity(0),JsonRequestBehavior.AllowGet);
     }