Which choice is better for making a vivid orange in watercolour?

Correct answer: Clean warm red and clean yellow

The governing concept is pigment bias in subtractive colour mixing. A clean warm red and a clean yellow contain little unwanted blue or green influence, so their mixture reflects a brighter orange. Blue-biased red or green-biased yellow can make the mixture dull, while red with green or yellow with violet introduces complementary contamination and produces a muted, brownish result. Therefore, option A is the best choice.