What is a likely result of adding excessive black to blue-green?

Correct answer: The colour may become too dark and lifeless

Black lowers the lightness of a colour and, when used excessively, can overpower the original hue. In blue-green, too much black may make the mixture very dark, dull, and visually lifeless rather than more saturated. Thus D is correct. The mixture does not become yellow-green or a primary colour, and adding black generally reduces rather than increases its vividness.