Check out example codes for "webforms validate radio buttons". It will help you in understanding the concepts better.

Code Example 1

<asp:RadioButtonList ID="RadioButtonList1" runat="server">
            <asp:ListItem>north</asp:ListItem>
            <asp:ListItem>west</asp:ListItem>
        </asp:RadioButtonList>
        <asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server"
            ControlToValidate="RadioButtonList1" ErrorMessage="RequiredFieldValidator">
        </asp:RequiredFieldValidator>

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