What is the correct way to gradually lighten colour in a gradation?

Correct answer: Add a little water or lighter colour at each step

Gradation depends on a controlled, progressive change in tone. Adding a small amount of water or a lighter colour at each successive stage creates several close values, so the dark-to-light transition appears smooth. Option A is therefore correct. A large amount of white produces an abrupt jump, black makes the mixture darker, and changing brush size alone does not change the colour value. Gentle blending between stages further improves the result.