A colour transition is technically smooth but appears too calm for the mood of the composition. How can tension be increased?
The governing concept is expressive use of edge and colour contrast. A composition gains visual tension when selected areas interrupt an otherwise calm flow with abrupt hue or value changes and deliberate hard boundaries. Option D is correct because it increases energy without flattening the entire image. More softness, isolated white, or universal greying would generally reduce contrast and make the mood calmer rather than more tense.