If equal amounts of red and yellow pigments produce a muddy colour instead of orange, what is the most likely cause?

Correct answer: Contamination by a third colour

The governing concept is subtractive pigment mixing. Red and yellow pigments normally produce an orange mixture when they are reasonably pure and applied with clean tools. A small amount of an unintended third pigment can absorb additional wavelengths and reduce colour clarity, making the result dull or muddy. Thus A is correct. Brightness, palette size, and white paper do not normally create this contamination effect.