What is a simple way to create a highlight with acrylic colour?

Correct answer: Apply white or a light colour

The governing concept is light and shade, especially the use of value to show reflected light. A highlight is created by applying white or a lighter version of the local colour to the area facing the light source. A dark layer produces a shadow, while adding oil or soaking the paper does not create a controlled highlight. Thus option C is correct; the light colour should be used sparingly for a natural effect.