What should be done to make a watercolour sky lighter toward the bottom?

Correct answer: Increase the amount of water in the colour mixture toward the bottom

The governing concept is a graded watercolour wash, in which pigment strength changes gradually across the surface. Increasing the water content toward the bottom dilutes the pigment, so the value becomes progressively lighter while the transition remains smooth. Option C is correct. Adding black would lower the value and make the area darker, while an equally strong wash would not create a lightening effect. Oil is unsuitable for this water-based technique.