Which contamination most strongly interferes with making a clean violet?

Correct answer: Yellow

The governing concept is the effect of complementary colours in pigment mixing. Clean violet is made from red and blue, and yellow is its complementary colour. Yellow contamination therefore reduces violet’s saturation and can make the mixture muddy or brownish. Option A is correct. White may produce a lighter tint, while black or grey may darken or dull the mixture, but they do not create the same direct complementary neutralisation described by the question.