/// <summary>
/// This WebMethod returns the XML for a DropDownList having value and text.
/// </summary>
/// <param name="tableName">string: Name of the Table having DropDownList items.</param>
/// <returns>string: XML containing all the items.</returns>
[System.Web.Services.WebMethod]
public static string GetDropDownItems(string tableName)
{
    // Create a dummy DataTable.
    DataTable dt = new DataTable(tableName);
    dt.Columns.Add("OptionValue");
    dt.Columns.Add("OptionText");
    dt.Rows.Add("0", "Item 0");
    dt.Rows.Add("1", "Item 1");
    dt.Rows.Add("2", "Item 2");
    dt.Rows.Add("3", "Item 3");
    dt.Rows.Add("4", "Item 4");

    // Convert the DataTable to XML.
    string result;
    using (StringWriter sw = new StringWriter())
    {
        dt.WriteXml(sw);
        result = sw.ToString();
    }

    return result;
}

function GetDropDownData() {
    // Get the DropDownList.
    var ddlTestDropDownListXML = $('#ddlTestDropDownListXML');
    
    // Provide Some Table name to pass to the WebMethod as a parameter.
    var tableName = "someTableName";

    $.ajax({
        type: "POST",
        url: "BindDropDownList.aspx/GetDropDownItems",
        data: '{tableName: "' + tableName + '"}',
        contentType: "application/json; charset=utf-8",
        dataType: "json",
        success: function (response) {
            // Now find the Table from response and loop through each item (row).
            $(response.d).find(tableName).each(function () {
                // Get the OptionValue and OptionText Column values.
                var OptionValue = $(this).find('OptionValue').text();
                var OptionText = $(this).find('OptionText').text();
                
                // Create an Option for DropDownList.
                var option = $("<option>" + OptionText + "</option>");
                option.attr("value", OptionValue);

                ddlTestDropDownListXML.append(option);
            });
        },
        failure: function (response) {
            alert(response.d);
        }
    });
}