Which colour is generally added to make another colour lighter?

Correct answer: White

The governing colour-mixing concept is the tint. When white is mixed into a hue, the resulting mixture becomes lighter while usually retaining the original hue family; this is called a tint. Therefore option D is correct. Black generally darkens a colour and produces a shade, while brown or green does not provide the general lightening rule asked here. In simple art theory, remember: colour plus white equals tint; colour plus black equals shade.