Which effect is useful for painting soft clouds in watercolour?

Correct answer: Wet-on-wet effect

The governing concept is using edge quality to suggest soft forms. In the wet-on-wet method, a lightly moistened surface lets diluted watercolour spread gently, creating blurred transitions suitable for clouds. Therefore, option C is correct. Hard black lines produce sharp outlines, a thick layer can look heavy, and oil mixture is not part of normal watercolour practice. A light wash can make the effect more natural.