Which technique is suitable for creating a sharp corner with a broad flat brush in poster colour?

Correct answer: Controlled use of the narrow edge of the brush

A flat brush has a narrow edge that can act like a precise line-making tool. By holding that edge steadily and changing direction carefully at the meeting point, the painter can form a clean, sharp corner. Therefore, option A is correct. Circular movement, excessive pressure, or dripping paint reduces control and usually produces blurred or uneven edges.