What is the most likely brush-related cause of small bubbles forming in a colour layer?

Correct answer: Whipping the paint too quickly and repeatedly with the brush

The governing concept is air entrainment during paint preparation. If colour is whipped rapidly and repeatedly with a brush, air becomes trapped in the liquid and appears as small bubbles when the layer is spread. Option D is therefore correct. A brush’s colour, the paper’s shape, or the fact that the brush is new does not normally explain bubbles. Gentle mixing helps prevent them.