What is the main cause of dark colour collecting at the edge of strokes while creating an even flat wash?

Correct answer: A drying edge and uneven moisture

In a flat wash, the governing principle is to maintain a connected, evenly moist edge. If one stroke begins drying and a fresh wet stroke is placed beside it, water movement can push pigment toward the boundary, producing a darker line or bloom. Therefore option B is correct. Paper size, handle weight and short bristles do not by themselves cause this edge-collection effect; uneven moisture and timing do.