Which colour can be useful in the middle of a smooth blend from blue to green?

Correct answer: Blue-green mixture

A smooth blend is made by introducing an intermediate colour that shares qualities with both endpoints. Blue-green lies between blue and green, so it can bridge the transition and reduce a sudden change in hue; option C is correct. Red and orange are not the nearest transition colours for this pair, while black would darken the mixture rather than create a clean intermediate hue.