1. Give runat="server" and Id to each Anchor Tag
<a runat = "server" id = "lnkDefault" href="Default.aspx" class="active">About</a>
2. Then in page load check URL and set Active class
protected void Page_Load(object sender, EventArgs e)
{
if (Request.Url.AbsolutePath.EndsWith("default.aspx"))
{
lnkDefault.Attributes["class"] = "active";
}
}
and write similar code for all Anchors