What is added to lighten a colour that has become too dark?

Correct answer: White

The governing concept is value adjustment through tinting. Adding white increases the lightness, or value, of a dark colour and creates a lighter version called a tint. Black would lower the value further, while brown or violet would mainly change the hue and may make the mixture duller rather than reliably lighter. Thus option D is correct; white should be added gradually to control the result.