What is a possible cause of small bubbles forming during colour filling?

Correct answer: Whipping the paint or scrubbing rapidly with the brush

The relevant concept is trapped air in the paint film. Vigorous whipping, rapid stirring, or aggressive scrubbing can introduce air and form small bubbles in the wet colour. Therefore option D is the plausible cause. A clean brush does not normally create bubbles, light colour only describes appearance, and a closed shape is a drawing condition rather than a cause of air entering the paint. Gentle mixing and controlled strokes reduce the problem.