What is a simple way to make light brown for a tree trunk in watercolour?

Correct answer: Add more water to brown

In transparent watercolour, the value or lightness of a colour is mainly controlled by the amount of water. Adding more clean water to brown spreads the pigment into a paler, lighter transparent wash, so option D is correct. Black would make the brown darker, oil is unsuitable for ordinary watercolour paper, and a thick layer generally increases darkness rather than producing a delicate light tone.