How can dark grey be made?

Correct answer: Add a little more black to grey

The governing concept is changing value by altering the black-to-white ratio. To make an existing grey darker, add a small amount of black and mix thoroughly; black lowers the lightness of the neutral mixture. Therefore option A is correct. Adding white would create light grey, while yellow or red would shift the hue away from neutral grey instead of simply darkening it. Small additions prevent the shade from becoming too dark too quickly.