What is generally added to red to make it darker?

Correct answer: A little black

The governing concept is the difference between a tint and a shade. Adding a small amount of black to red makes a darker red, called a shade, so option D is correct. White would create a lighter red or tint. Yellow would move the colour toward orange, while green, the complementary colour of red, may neutralise it and produce a dull or muddy mixture.