Which of the following is not a secondary colour?

Correct answer: Red

Option D, red, is not a secondary colour; it is one of the three primary colours in the traditional pigment colour system. Orange, green, and violet are secondary colours because each is made by mixing a pair of primary colours: red with yellow, yellow with blue, and red with blue respectively. Therefore, identifying the colour that can serve as a starting primary pigment makes red the only correct answer.