How to make radiobutton checked on dropdown selectedindex chaged event in mvc 4?

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MVC4

entity-framework

This is my view code
@using (Html.BeginForm())
{
@Html.LabelFor(m=>m.GroupID)
@Html.DropDownListFor(m => m.GroupID, Model.GroupList, "Please select", new { id = "ddlgrp" })
if (Model==null)
{

foreach(var permission in Model.Permissions)
{
<label>
@Html.RadioButtonFor(m=>m.perm_id,permission.perm_id)
@permission.perm_levelname
</label>
}
}
else
{

foreach(var permission in Model.Permissions)
{
<label>
@if (Model.perm_id.Equals(ViewBag.selectedperms))
{


@Html.RadioButtonFor(m=>m.perm_id,permission.perm_id,true)
@permission.perm_levelname
}
else
{
@Html.RadioButtonFor(m=>m.perm_id,permission.perm_id)
@permission.perm_levelname
}
</label>
}
}

<input type="submit" value="Submit" />

This is my jquery code

$(document).ready(function () {
        $("#ddlgrp").change(function () {

            $("#log").ajaxError(function (event, jqxhr, settings, exception) {
                alert(exception);
            });
            var grpselected = $("select option:selected").val();
            alert(grpselected);
  $.get('@Url.Action("CheckPermissions")',
            { id: grpselected }, function (data) {
              });
            });
    });

This is actionmethod

public ActionResult CheckPermissions(int id)
        {
            tblperm od = new tblperm();
            var selectedperms = (from c in db.tblperm where c.grp_id==id select c.perm_id).SingleOrDefault();
            ViewBag.selectedperms = selectedperms;
            return View(od);
        }

Assume for groupip id 1 there is a permisson with permid 1. So when i select 1 from dropdwon corresponding dropdown with id 1 should be checked. I have tried with above code but its not working. Can anybody suggest me where i am going wrong?

What I have tried:

I want to make radiobutton checked based on the value from database