John works as a Web Developer for ProMetaworks Inc. He creates an ASP.NET application, named MyApp1, by using Visual Studio .NET. MyApp1 contains a page, named Page1. Page1 contains a RadioButtonList control that contains five items. John wants to ensure that users select one of the items from the RadioButtonList control before submitting the page. He wants to accomplish the task with least development effort. Which of the following validation controls will he use to accomplish the task?