function CustomValidator1_ClientValidate(source, args) {
  if (document.getElementById("<%= r1a.ClientID %>").checked || document.getElementById("<%= r1b.ClientID %>").checked || document.getElementById("<%= r1c.ClientID %>").checked || document.getElementById("<%= r1d.ClientID %>").checked || document.getElementById("<%= r1e.ClientID %>").checked) {
    args.IsValid = true;
  } else {
    args.IsValid = false;
  }

}

<table>
  <tr>
    <td>
      <asp:CustomValidator id="CustomValidator2" runat="server" Display="Dynamic" ForeColor="Red" ErrorMessage="Select one option" ClientValidationFunction="CustomValidator1_ClientValidate">Select one option</asp:CustomValidator>

    </td>
    <td class="auto-style2" style="vertical-align: middle; text-align: center;" text-align: center; "> </td>
    <asp:RadioButton ID="r1a " runat="server " GroupName="r1 " OnCheckedChanged="r1a_CheckedChanged " AutoPostBack="true " />
    </td>
    <td style="vertical-align: middle; text-align: center; ">
      <asp:RadioButton ID="r1b " runat="server " GroupName="r1 " OnCheckedChanged="r1b_CheckedChanged " AutoPostBack="true " />
    </td>
    <td style="vertical-align: middle; text-align: center; ">
      <asp:RadioButton ID="r1c " runat="server " GroupName="r1 " OnCheckedChanged="r1c_CheckedChanged " AutoPostBack="true " />
    </td>
    <td style="vertical-align: middle; text-align: center; ">
      <asp:RadioButton ID="r1d " runat="server " GroupName="r1 " OnCheckedChanged="r1d_CheckedChanged " AutoPostBack="true " />
    </td>
    <td style="vertical-align: middle; text-align: center; ">
      <asp:RadioButton ID="r1e " runat="server " GroupName="r1 " OnCheckedChanged="r1e_CheckedChanged " AutoPostBack="true " />
    </td>
  </tr>
  </tr>
</table>