Which mixture can produce a cool grey?

Correct answer: Black and white with a little blue

The governing concept is colour temperature controlled by an undertone. Black and white first create a neutral grey, and adding a small amount of blue shifts that grey toward a cool appearance, so option A is correct. Red would introduce warmth, while yellow and orange or red and yellow are mixtures of warm hues and do not form a cool grey base. Only a little blue is needed; too much would make the mixture distinctly blue rather than grey.