What is formed by adding black to the primary colour red?

Correct answer: A shade of red

Colour theory defines a shade as a hue mixed with black. Adding black to red lowers its lightness or value and produces a darker red while retaining red as the underlying hue. A tint would require white, not black; the complement of red is green in the traditional colour wheel, and the mixture cannot create pure yellow. Thus, option B is the correct answer.