Which mixture can produce a light blue colour?

Correct answer: Blue and white

The governing concept is the formation of a tint. When white is mixed into a hue such as blue, the colour becomes lighter and brighter while retaining its blue character; the result is called a light-blue tint. Therefore, option D is correct. Black would darken blue, red would move it toward violet, and yellow would move it toward green. The result may vary in strength according to the amount of white used.