Hi behnam haji,
As you asked, what is the C# class for td tag of html?
So you can cast the td tag in C# by writing the code given below.
System.Web.UI.HtmlControls.HtmlTableCell atag = rpt.FindControl("id of a tag") as System.Web.UI.HtmlControls.HtmlTableCell;
using System.Web.UI.HtmlControls;
HtmlTableCell atag = rpt.FindControl("id of a tag") as HtmlTableCell;
and you can also see more html control classes from
HtmlControls namespace.