Which colour can be added to make orange darker?

Correct answer: A small amount of black

The governing concept is the difference between a shade and a tint. Adding a small amount of black reduces the lightness of orange and produces a darker orange shade, so option A is correct. White would create a lighter tint, and extra yellow would shift orange toward yellow-orange rather than simply darkening it. Blue, the complementary colour of orange, may neutralise the mixture and make it dull or brownish, so it is not the best answer for a clean darker orange.