Why may an orange mixture turn brownish?

Correct answer: A trace of blue entered it

The governing concept is complementary-colour contamination. Orange is formed by mixing red and yellow. If even a small amount of blue enters the mixture, blue acts as orange’s complement and absorbs or neutralises some of its vividness. The result may look dull, brownish, or muddy. Clean red and yellow paints, a new brush, and a clean palette would reduce contamination rather than cause it, so A is correct.