Which mixture may be useful for creating a natural shadow of red in watercolour?

Correct answer: Adding a small amount of green to red

The governing concept is complementary colour mixing. Green is opposite red on the conventional colour wheel, so adding a very small amount of green reduces red’s saturation and can create a muted, neutral shadow colour. The quantity must be controlled because too much green may produce a muddy result. White mainly creates a tint, yellow shifts the hue toward orange, and extra water makes a lighter transparent wash rather than a natural dark shadow. Thus option A is correct.