<asp:Radiobuttonlist id="RadioButtonList1" runat="server" repeatdirection="Horizontal" autopostback="True" onselectedindexchanged="RadioButtonList1_SelectedIndexChanged" xmlns:asp="#unknown">
<asp:listitem enabled="true" value="Airtel">Airtel</asp:listitem>
<asp:listitem enabled="true" value="IDEA">IDEA</asp:listitem>
<asp:listitem enabled="true" value="Docomo">Docomo</asp:listitem>
<asp:listitem enabled="true" value="Other">Other</asp:listitem></asp:Radiobuttonlist>
on RadioButtonList1_SelectedIndexChanged Event Handle the textbox.
protected void RadioButtonList1_SelectedIndexChanged(object sender, EventArgs e)
{
if (string.Equals(RadioButtonList1.SelectedValue,"Other"))
{
TextBox1.Visible = true;
}
else
{
TextBox1.Visible =false;
}
}