How can I do this for DropDownList in MVC5?

Sir Ranjeet,

Here is my code for controller class:-

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;

namespace OrganisationDemo.Controllers
{
public class HomeController : Controller
{
Entity _entities = new Entity();

public ActionResult Index()
{
ViewBag.OrganisationList = new SelectList(_entities.Organisations.ToList(), "OrganisationID", "OrganisationName").FirstOrDefault();
return View();
}

public JsonResult GetDepartment(int organisaionId)
{
var department = (from dept in _entities.Departments
where dept.DepartmentID == organisaionId
select dept.DepartmentName).ToList();

return Json(new SelectList(department.ToArray(), "DepartmentID", "DepartmentName"), JsonRequestBehavior.AllowGet);
}

public JsonResult GetGrade(int departmentID)
{
var gradeList = (from grade in _entities.Grades
where grade.DepartmentID == departmentID
select grade.GradeName).ToList();

return Json(new SelectList(gradeList.ToArray(), "GardeID", "GradeName"), JsonRequestBehavior.AllowGet);
}

public ActionResult About()
{
ViewBag.Message = "Your app description page.";

return View();
}

public ActionResult Contact()
{
ViewBag.Message = "Your contact page.";

return View();
}
}
}
--------------------------------------------------------------------------------\
And here is my view code:-

@model OrganisationDemo.Entity

@{
ViewBag.Title = "Home Page";
}

@using (Html.BeginForm("Index", "Home", FormMethod.Post))
{
<div>Select Organisation : </div>
@Html.DropDownList("organisation", new SelectList(ViewBag.OrganisationList), new { id = "drporganisation"})
<br />
<select id="txtdepartment" name="department"></select>
<br />
<select id="txtgrade" name="grade"></select>
<br />
}

@section scripts
{
<script type="text/javascript">
$(document).ready(function () {
$("#txtdepartment #txtgrade").prop("disabled", true);
$("#drporganisation").change(function () {
var options = {};
options.url = "home/getdepartment";
options.type = "POST";
option.data = JSON.stringify({ organisaionId: $("#drporganisation").val() });
options.datatype = "json";
options.contentType = "application/json";
options.success = function (department) {
$("#txtdepartment").empty();
for (var i = 0; i < department.length; i++)
$("#txtdepartment").append("<option>" + department[i] + "</options>");
}
$("#department").prop("disabled", false);
$.ajax(options);
});
});
</script>
}
--------------------------------------------------------------

What could be the wrong with my code?I am unable to retrieve the any of the drop down.

Please help me.

Sorry Sir, my requirement has been changed and I was not informed.Apologies.

or you want to select the organization and then the department will fill then selection on the Department the Grade should be coming in the dropdown. (if this is the requirement then your action method are quite correct)

This i want. But getting error in view section.