"rblComplaint" runat="server" OnSelectedIndexChanged="rblComplaint_SelectedIndexChanged"
                RepeatDirection="Horizontal">
                                Yes
                                No
                            
                            "LinkButton1" runat="server">LinkButton
                            "mpeComplaint" runat="server">
                                "btnCancel" OkControlID="btnOK" PopupControlID="Panel1" TargetControlID="LinkButton1" BackgroundCssClass="modalBackground" />
                            
                            
                            "Panel1" runat="server" CssClass="modalPopup">
                                Details:"txtDetails" runat="server">
                                 

                                "btnOK" runat="server" Text="OK" />
                                "btnCancel" runat="server" Text="Cancel" />
                            

use the HtmlControl version of the radio buttons instead of a RadioButtonList

<input type="radio" value=""Yes" id="rbYes" runat="server" name="YesNo" />

<input type="radio" value=""No" id="rbNo" runat="server" name="YesNo" />

On the popup extender, set the TargetControlID to whatever the radio button you want to open the popup.