Which method can be useful for painting a sky in watercolour?

Correct answer: Wet-on-wet

The wet-on-wet method means applying watercolour to damp paper or adding a wash while the previous area is still moist. Because the water carries pigment gradually, it produces soft transitions and blurred edges that suit clouds, atmospheric sky, and gentle colour changes. Stone cutting, collage pasting, and kneading clay are different artistic processes and cannot create this characteristic watercolour blending. Therefore option A is correct.