Which practice is useful for maintaining equal letter width with poster colour?

Correct answer: Keep the angle of the flat brush constant

The governing skill is controlled brush lettering. A flat brush produces a more predictable stroke when its broad face is held at a constant angle, so the letters receive a similar width and weight. Option B is therefore correct. Changing brushes unnecessarily does not guarantee uniformity, rotating the brush changes the stroke width, and excess water can cause bleeding and uneven edges.