What should be done to reduce a hard line between two colours?

Correct answer: Use gentle back-and-forth strokes along the boundary

A hard boundary can be softened by using gentle back-and-forth strokes where the two colours meet. This gradually blends the transition without creating a new harsh mark, so option A is correct. A black line would make the boundary stronger, while cutting the paper or erasing all the colour removes the artwork rather than blending it. Light pressure is important for a controlled transition.