If a yellow-green mixture appears muddy, what type of error should be checked first?

Correct answer: Unwanted contamination by red or violet

A muddy appearance usually suggests that an unintended neutralising or complementary colour has entered the mixture. For yellow-green, red or violet contamination should therefore be checked first, along with the cleanliness of the brush and palette. Excess yellow or green mainly changes the hue balance, while white paper does not by itself contaminate the paint. Thus A is the best answer.