What is the most suitable way to lighten colour in transparent watercolour?

Correct answer: Add more water

Transparent watercolour depends on the amount of water and the whiteness of the paper rather than on thick white pigment. Adding more clean water spreads the pigment more thinly, so the colour becomes lighter and remains transparent. Thus option A is correct. Black would darken the mixture, thick white would reduce transparency, and rubbing dry pigment is not the normal method for lightening a wet watercolour wash. Excess water should still be controlled.