The absence of which colour is especially important for producing a clean orange from red and yellow?

Correct answer: Blue

The governing concept is complementary-colour contamination in pigment mixing. Red and yellow should combine to make orange, while blue is complementary to orange and can neutralise it, producing a dull or brownish result. White changes value by making a tint, black darkens the mixture, and green may also affect the hue, but blue is the especially important contaminant to avoid for a clean orange. Hence option A is correct.