Which brush is used to make very thin long lines?

Correct answer: Liner brush

The governing concept is selecting a brush according to the required line quality. A liner brush has long, narrow bristles that can hold a small amount of paint and release it in a controlled continuous mark, so option D is correct. A mop brush is soft and broad, a fan brush spreads marks, and a flat brush is better for broad strokes or edges than very thin long lines.