Which colour may be added in a very small amount to make green less bright and more natural?

Correct answer: Red

The governing concept is reducing intensity with a complementary colour. Red is opposite green on the colour wheel, so a very small amount of red neutralises some of green’s brightness and produces a more subdued, natural green. Thus B is correct. Yellow or blue shifts the green toward another hue, while white creates a lighter tint but does not effectively reduce saturation in the intended way.