How can colour be prevented from spreading outside the boundary while filling?

Correct answer: By using small controlled strokes

The governing concept is precise boundary control during colour application. Small, controlled strokes let the artist reduce pressure and guide the brush accurately near the outline, so the paint remains inside the shape. Therefore, option D is correct. Excessive water makes paint run, very fast strokes reduce accuracy, and closing the eyes removes visual control. Careful movement near edges is especially important for a neat finish.