Which colour is suitable for muting violet slightly?

Correct answer: Yellow

The governing concept is complementary colour mixing. Yellow is opposite violet on the colour wheel, so adding a very small quantity of yellow reduces violet’s saturation and makes it less intense. Option D is therefore correct. Red or blue would tend to shift violet toward a warmer or cooler violet, not neutralise it. Black can darken violet, but darkening is different from carefully muting its colour intensity. The amount should be added gradually to avoid an overly muddy result.