<asp:dropdown id=ddlDropdown runat=server/>

on page load
ddlDropdown.Attributes.Add("onchange","callMethod();")
<script langauge=javascript type="text/javascript">
function callMethod()
{

var pkval = Document.getElementbyId("ddlDrop").value;
PageMethods.FindData(pkVal,OnSucceeded,OnFailed);//finddata is serverside static method.
}
function OnSucceeded(result)
    {
         FillData(result);        
     }
    
     
    function OnFailed(error, userContext, methodName) 
    {
        
 alert("failure")  ;   
    }
</script>

  [WebMethod]//must be there or without adding using System.Web.Services; you directly add here
[System.Web.Services.WebMethod]
public static string RetriveData(string args)
        {
  int id = convert.Toint32(args);

//create a dataset use sql query like (where pkid = id)from another table

// remember the column will be your xml node so you can rename in your query like like pk_primary as pk
 DataSet dsData= //add code as your requirement
//after creating dataset convert to xml
 dsData.DataSetName = "CONTROLS";
            XmlDataDocument xDoc = new XmlDataDocument(dsData);
            string xml = xDoc.OuterXml;
            xml = xDoc.InnerXml;
            return xml;


}

result xml file does to here
function OnSucceeded(result)
    {
         FillData(result); // call another function       
     }

function FillData(xmlVal)
{
  //find the node values 
  var doc=new ActiveXObject("Microsoft.XMLDOM"); 
        doc.async="false"; 
        doc.loadXML(result); 
  var controls = doc.getElementsByTagName('Table');
 for(var i = 0; i < controls.length ; i++)
            {
var Contol_type = controls[i].getElementsByTagName('columnname as you assign in query in serverside')[0].childNodes[0].nodeValue;

//assign that Contol_type  to a another control or dropdown
            }
}