To answer this question, the user needs to have knowledge about the ways to activate a Button control in a Windows Forms application.
Option A is correct. A Button control can be activated programmatically by calling its Click event.
Option B is correct. A Button control can be activated by clicking on it with the mouse.
Option C is correct. A Button control can be activated by setting the form's DefaultButton property to the Button control that should be activated when the user presses the Enter key.
Therefore, option D is correct since both option A and B are correct.
Option E is incorrect since it is not necessary to use all three methods to activate a Button control.