Why is green contamination especially harmful while making clean red-orange?

Correct answer: Green is complementary to red and can mute the mixture

Green lies opposite the red region on the conventional colour wheel, so it has a complementary relationship with red. When green accidentally enters a red-orange mixture, it can neutralise some of the red influence and reduce chromatic intensity, producing a dull or muddy result. Hence D is correct. Green is not outside the wheel and is not simply a light form of red.