What is a simple way to lighten a very dark wet area in watercolour?

Correct answer: Lift some colour with a clean, damp brush

The governing concept is pigment lifting in transparent watercolour. While the area is still wet, touch it gently with a clean, damp brush, allow the brush to absorb some pigment, rinse it, and repeat carefully if needed. Removing pigment makes the value lighter. More concentrated colour or another dark wash would increase darkness, while oil is unsuitable for watercolour. Therefore, option A is correct.