Which useful colour variation is obtained by adding white to violet?

Correct answer: Light violet

Violet is a secondary colour, and adding white increases its lightness while preserving its basic hue. The resulting lighter variation is called a violet tint and is commonly described as light violet. White does not create a different hue such as blue-green or red-orange, and dark orange would require a separate orange mixture and darkening process. Therefore, option A is the only correct answer.