Which colour is useful for slightly muting bright blue?

Correct answer: Orange

The governing concept is complementary-colour neutralisation. Orange is opposite blue on the colour wheel, so adding a very small amount of orange reduces blue’s saturation and makes it look less bright or more muted. The amount must be controlled because too much orange can produce a muddy brown or grey mixture. White lightens blue but does not mute it in the same way; green and violet are neighbouring or related hues, not the standard complement. Thus option D is correct.