Which correction is suitable for making an overly vivid green look natural in watercolour?

Correct answer: Add a tiny amount of red

The governing concept is saturation control through a complementary colour. Red is complementary to green, so adding only a tiny amount reduces the green’s excessive chroma and makes it look more natural. More yellow or blue mainly shifts the hue toward yellow-green or blue-green, while extra water makes the wash lighter and more transparent rather than truly neutralising it. Too much red would create a dull brown or muddy mixture, so option C is correct.