When does a dry-brush-like rough effect occur in a flat poster-colour layer?

Correct answer: When both moisture and paint on the brush are insufficient

The governing concept of dry-brush technique is incomplete paint transfer. When the brush contains too little moisture and too little colour, its hairs skip over the surface and leave broken, scratchy marks instead of an even film. Therefore, option A is correct. Suitable water and a fully loaded brush generally support smoother coverage, while paper smoothness alone is not the defining cause. A suitably moist, evenly loaded brush is better for flat areas.