The three Visualforce components that can be used to initiate Ajax behavior to perform partial page updates are apex:commandLink, apex:commandButton, and apex:actionSupport. These components allow the developer to invoke an Apex controller method or a JavaScript function, and specify a apex:actionStatus or a apex:reRender attribute to update a part of the page without refreshing the whole page. The apex:commandLink and apex:commandButton components render as HTML links or buttons, respectively, and can be used to trigger an action when the user clicks on them. The apex:actionSupport component can be used to add Ajax support to any other component, and can be used to trigger an action when the user performs an event, such as mouseover, change, or blur. The apex:form component is not a valid answer, as it is used to enclose the components that need to interact with the server, but it does not initiate Ajax behavior by itself. The apex:actionStatus component is not a valid answer, as it is used to display the status of an Ajax request, such as a loading message or a spinner, but it does not initiate Ajax behavior by itself. Reference: [Visualforce Components Reference], [Visualforce Developer Guide]