Which mixture is suitable for adding depth to sunset orange?

Correct answer: Add a little red and a very small amount of violet to orange

The governing concept is controlled adjustment of hue, value, and saturation. A little red makes orange warmer and richer, while a very small amount of violet, a cool neighbouring/complementary influence, can reduce excessive brightness and suggest depth. Option A is therefore suitable. More white makes a lighter tint, water mainly makes a transparent wash, and much green can neutralise or muddy the orange instead of creating a warm sunset depth.