The correct answer is: C. Can be selected by the Enter key even when not focused.
A default button is a button that is preselected when a form is opened. It is usually the button that is used to submit the form. The default button can be selected by pressing the Enter key, even when it is not focused. This means that the user does not have to click on the button to select it.
The other options are incorrect. Option A is incorrect because the default button is not always focused by default. Option B is incorrect because the default button does not always clear the form it is in. Option D is incorrect because the default button cannot be selected by the Shift key even when it is not focused.