Which contamination most strongly interferes with making a clean violet from red and blue?

Correct answer: Yellow

The governing concept is keeping a secondary pigment mixture limited to its required primaries. Red and blue make violet; adding yellow introduces the third primary colour and tends to neutralise the mixture, making it muddy or brownish instead of clean violet. White produces a lighter tint, black produces a darker shade, and grey lowers brightness, but yellow most directly contaminates the intended three-primary balance. Therefore option A is correct.