The brush speed is constant but stroke transparency keeps changing. Which factor is most likely responsible?

Correct answer: A changing ratio of water to paint in the brush

The governing concept is pigment concentration: more water relative to paint produces a thinner, more transparent mark, while more pigment produces a denser, less transparent mark. Therefore, even with constant brush speed, changing the water-to-paint ratio changes the stroke’s opacity. Handle colour, the paper’s name and the subject do not directly alter the paint concentration in the loaded brush. Option C is correct.