What is the correct way to remove excess paint collected at the bottom of a watercolour wash?

Correct answer: Lift the excess with a damp clean brush

The governing technique is controlled lifting in watercolour. A clean, slightly damp brush can absorb the surplus pigment from the bottom edge without adding a new colour or damaging the paper. Therefore option A is correct. Rubbing may roughen or tear the paper, adding black makes the area darker, and washing the whole region can spread the excess paint and destroy the intended wash. The brush should be damp, not dripping wet.