The Button control can be activated:
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programmatically through the click event.
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by clicking the button with the mouse.
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with the form’s DefaultButton property.
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Both a and b.
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All of the above.
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.
Therefore, the answer is: D.
A Button control responds to a click both when the user physically clicks it with the mouse and when code raises the click event programmatically (e.g., calling Button1_Click or invoking PerformClick) — both trigger the same event handler. The DefaultButton property is a separate mechanism (it fires the button's click when Enter is pressed on the form), so it isn't one of the two ways described here.