Which tertiary colour can be chosen to add more redness to a violet flower?

Correct answer: Red-violet

The governing concept is hue bias within a colour mixture. To make a violet flower appear redder, select a tertiary colour that contains both violet and a noticeable red influence. Red-violet satisfies this requirement and preserves a relationship with the flower’s original violet hue. Blue-violet would increase the blue influence, while yellow-green and blue-green would introduce unrelated green components. Therefore, option A is correct.