What should be added to make grey darker?

Correct answer: Black

The governing concept is adjustment of value, or lightness and darkness. Adding a small amount of black lowers the value of grey and makes it appear darker, so option D is correct. White would raise the value and create a lighter grey. Yellow or pink would mainly alter the hue and could produce a coloured grey, but neither is the standard answer for simply darkening it. Pigment should be added gradually to avoid making the mixture too dark.