What may be added in a small amount to darken a colour?

Correct answer: Black

The governing concept is shade formation in colour mixing. Adding a very small amount of black lowers the lightness of the original colour and produces a darker shade, so option C is correct. White would make the colour lighter, water mainly changes paint fluidity or transparency, and yellow changes the hue rather than reliably darkening it. Black should be added gradually because too much can make the mixture dull or muddy.