When is adding a complementary colour better than adding black?

Correct answer: When brightness must be reduced without greatly lowering value

The governing concept is the difference between chroma and value. A complementary colour mainly neutralises or reduces brightness and saturation, whereas black lowers value and makes a colour darker. Therefore, if the aim is to mute an overly bright colour without greatly darkening it, the complementary mixture is more suitable. White lightens, water affects transparency, and black is appropriate for a dark shade. Option D is correct.