Which is the most useful time for creating a soft blend with acrylic colour?

Correct answer: While the paint is still wet

Soft blending depends on allowing neighbouring colours to merge before the paint film sets. With acrylic colour, the most useful moment is while both colours are still wet, because the brush can soften the boundary and create a gradual transition. Thus option B is correct. Once the paint has dried, direct blending becomes difficult and may require another thin layer. Varnishing or folding the canvas does not create a colour blend and may damage the work.