If a colour becomes too dark what is generally added to lighten it?

Correct answer: White

The governing concept is tinting: adding white produces a lighter version of a colour. Therefore, option D is correct. White raises the apparent lightness while largely preserving the original hue when used carefully. Black would deepen the colour further, and brown or blue would mainly alter its hue and may make it duller rather than simply lighter. In practice, white should be added in small amounts because excessive white can make the mixture chalky.