How can an overly vivid orange poster colour be muted in a controlled way?

Correct answer: By adding a tiny amount of blue

The governing concept is complementary colour neutralisation. Blue is opposite orange on the colour wheel, so adding a tiny amount of blue lowers orange’s saturation and produces a more muted, balanced tone. Thus option D is correct. Adding more yellow or red generally strengthens or shifts the orange, while a large amount of white mainly raises its value and makes it lighter rather than truly neutralising its intensity. Add blue gradually because excess can make the colour muddy.