The action method invoked by the AJAX request to the server. Use merge-field syntax to reference the method. For example, action="{!save}" references the save() method in the controller. If an action isn't specified, the page simply refreshes. Note that command links associated with the save, edit, or delete actions in a standard controller are rendered only if the user has the appropriate permissions. Likewise, command links associated with the edit and delete actions are rendered only if a record is associated with the page.

D is the correct answer,