What is the benefit of adding a complementary colour instead of only black to create a shadow in poster colour?

Correct answer: The shadow may appear richer and more natural

The governing concept is complementary-colour mixing in shadow modelling. A small amount of the complementary colour lowers the intensity of the original hue and can create a deeper, more varied shadow while preserving colour character. By contrast, black alone may make the mixture look flat or dull, although it can darken it. Too much complementary colour can also muddy the mixture. Thus option A is the precise benefit.