What is the most direct way to darken blue?

Correct answer: Add a little black

The governing concept is the formation of a shade: adding a small amount of black lowers the lightness of the original colour, so blue becomes darker and option D is correct. White would create a lighter tint. Yellow would shift blue towards green, while orange, its complementary colour, can reduce saturation and produce a dull or brownish mixture. Black should be added gradually because too much can overpower the blue.