Correct Answer : ScriptManager
Explanation :
ScriptManager control is mandatory control if you want to work with ASP.NET AJAX server control. Only one instance of a ScriptManager can be added to the web page. It manages the JavaScript files sent to the client and the communication between the server and the client. It also manages partial-page updates. The HTML for a ScriptManager control in Source view looks as follows:
< asp:UpdatePanel ID="UpdatePanel1" runat="server">
