What is useful for creating a soft transition between two watercolours?

Correct answer: Blend both colours gradually on a slightly wet surface

The governing technique is controlled wet-on-wet blending. On a slightly damp surface, water allows the edges of neighbouring pigments to move and merge gradually, producing a soft transition rather than a sharp boundary. Therefore, option A is correct. A black line creates separation, completely dry paper usually preserves a hard edge, and sand is not a normal blending material for watercolour. The brush still needs controlled moisture to prevent uncontrolled spreading.