What is necessary for making a thin straight line with acrylic colour?

Correct answer: A thin brush and light even pressure

The governing technique is controlled mark-making. A thin brush places a narrow stroke, while light, even pressure keeps its width and direction stable as the hand moves. Hence option C is correct. A broad, very wet brush produces a thick or uncontrolled mark; oil on paper interferes with the surface and paint adhesion; and excess paint can create blobs rather than a clean line.