What should be done to change a soft watercolour edge into a clear edge later?

Correct answer: Define the edge with controlled paint after the surface dries

The governing concept is the contrast between soft-edge and hard-edge watercolour work. A soft edge is produced when wet paint merges into a damp area. To sharpen it, first allow the paper and paint to dry; then use a controlled load of paint and a suitable brush to redraw the boundary. Thus, option B is correct. Adding water, rubbing with cloth, or dipping the paper would disturb the pigment and make the edge less controlled. This is an example of wet-on-dry refinement.