What is the best way to slightly mute a very bright blue-green?

Correct answer: Add a little red-orange

The governing concept is complementary colour neutralisation. Red-orange is opposite blue-green on the colour wheel, so a small amount reduces the colour’s chromatic intensity and makes it less brilliant. Option D is correct, but it must be added gradually because too much complementary pigment can produce a dull brown or grey. More blue or green increases saturation, while much white creates a tint rather than a controlled muted colour.