function FillCity() {
        var stateID = $("#ddlState").val();

        $.ajax({
            url: '@Url.Action("Employee", "Index")',
            type: "GET",
            dataType: "json",
            data: { state: stateID },
            async: true,
            processData: true,
            success: function (data) {
                alert("SSSS");
            },
            error: function (data) {
                alert("error");
            }
        });
    }

public ActionResult FillCity(int state)
    {
        var cities = subbuObject.Cities.Where(c=>c.StateID == state);
        return Json(cities, JsonRequestBehavior.AllowGet);
    }

$.ajax({
    url: '@Url.Action("YourActionNamne", "YourControllerName")',
    type: "GET",
    dataType: "json",
    data: { state: stateID },
    async: true,
    processData: true,
    success: function (data) {
        alert("SSSS");
    },
    error: function (data) {
        alert("error");
    }
});

url: '@Url.Action("FillCity", "Employee")'

url: '@Url.Action("ActionName", "ControllerName", new { Area = "AreaName" })',

public class EmployeeController: Controller
{
        [HttpGet]
        public ActionResult FillCity(int state)
        {
             var cities = subbuObject.Cities.Where(c=>c.StateID == state);
            return Json(cities , "application/json", JsonRequestBehavior.AllowGet);
        }
}

<script>
        function fillCity() {
            var stateID = $("#ddlState").val();

            $.ajax({
                url: '@Url.Action("FillCity", "Employee")',
            type: "GET",
            dataType: "json",
            data: { state: stateID },
            success: function (data) {
                alert("SSSS");
            },
            error: function (data) {
                alert("error");
            }
        });
    }
    </script>