Which mixture can make yellow appear relatively cooler?

Correct answer: Adding a little green to yellow

The governing concept is relative colour temperature produced by hue bias. Adding a small amount of green shifts yellow toward yellow-green, which is generally treated as the cooler side of yellow. Red or orange shifts yellow toward a warmer yellow-orange direction. Brown mainly dulls or darkens the colour and does not provide as clear a cool shift. Therefore option B is the best answer.