What can be done while wet if a colour area becomes darker than intended?

Correct answer: Lift some colour with a clean damp brush

The governing concept is colour lifting in watercolour. While the paint remains wet, a clean, damp brush can absorb some excess pigment from the dark area; the brush should be blotted first so it does not simply spread the colour. This lightens the tone gradually. Applying black makes it darker, scratching damages the paper fibres, and folding cannot correct the paint. Therefore, option B is correct.