EDIT:
Okay, so after reviewing the problem more the issue is that you're handling events on the client and in code behind, which means that you're client-side handlers are cleared because the page performs a post-back. Your code is more targeted towards a stateful postback rather than a stateless AJAX setup, so in that light we'll move the logic to the code-behind.
First, clear out the script block in the head, it will not help you with anything. We're going to put it's login into ddlCourse_SelectedIndexChanged().
Next, we're going to edit ddlCourse_SelectedIndexChanged():
protected void ddlCourse_SelectedIndexChanged(object sender, EventArgs e)
{
if(ddlCourse.DataTextField == "MCA")
{
t1.Visible = true;
t2.Visible = true;
t3.Visible = true;
t4.Visible = false;
t5.Visible = false;
t6.Visible = false;
}
...
}
Last, modify your HTML so that the tags are rendered by the server:
<li id="t1" runat="server"><a href="#tabs-1">Tab 1</a></li>
<li id="t2" runat="server"><a href="#tabs-2">Tab 2</a></li>
<li id="t3" runat="server"><a href="#tabs-3">Tab 3</a></li>
<li id="t4" runat="server"><a href="#tabs-4">Tab 4</a></li>
<li id="t5" runat="server"><a href="#tabs-5">Tab 5</a></li>
<li id="t6" runat="server"><a href="#tabs-6">Tab 6</a></li>