Which technique is useful for painting a sky in watercolour?

Correct answer: Wet-on-wet

The governing concept is wet-on-wet watercolour technique. In this method, moist paint is applied to damp paper or to a still-wet wash, allowing nearby colours to spread and merge gradually. That controlled softness is useful for broad sky areas, clouds, and gentle tonal transitions. Dry brush, black outlines, and cutting paper do not create the same fluid atmospheric effect. Thus option D is the most suitable answer.