As you are using WebForms the easiest is to change your ASPX to:
<label ID="myLabel" runat="server" ClientIDMode="Static" for="Rdb_1">No</label>
and then in code behind:
myLabel.InnerText
However I would suggest you investigate using a label control as that would be even easier to use.
solved How to get text value of HTML label in c# in ASP.NET WebForms [closed]