Which method is useful for making a flat sky background with poster colour?

Correct answer: Use enough mixed colour and a broad brush

The governing technique for a flat background is even application of a consistent colour. Preparing enough well-mixed poster colour prevents interruptions, and a broad brush covers a large area with fewer visible strokes. Thus, option A is correct. Making different blues creates uneven sections, tearing the paper damages the surface, and adding sand is unsuitable for a smooth sky background. The brush should be loaded evenly and moved in controlled strokes.