Radio Button Group

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="radiobut_checked.aspx.cs" Inherits="Forum_Topics_radiobut_checked" %>

using System;

using System.Collections;

using System.Configuration;

using System.Data;

using System.Linq;

using System.Web;

using System.Web.Security;

using System.Web.UI;

using System.Web.UI.HtmlControls;

using System.Web.UI.WebControls;

using System.Web.UI.WebControls.WebParts;

using System.Xml.Linq;

public partial class Forum_Topics_radiobut_checked : System.Web.UI.Page

{

    protected void Page_Load(object sender, EventArgs e)

    {

    }

    protected void btnSubmit_Click(object sender, EventArgs e)

    {

        if (rannually.Checked)

            lblmsg.Text = "rannualy is selected";

        if(rMonthly.Checked)

            lblmsg.Text = "rMonthly is selected";

        if(rQauterly.Checked)

            lblmsg.Text = "rQauterly is selected";

    }

}

For detail you check the link also
https://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.radiobutton.groupname(v=vs.110).aspx