Which technique is most suitable for the rough texture of tree bark in watercolour?

Correct answer: Broken strokes with a low-moisture brush

A low-moisture or dry brush carries relatively little liquid, so it skips across the raised tooth of the paper and leaves irregular broken marks. Those interrupted strokes imitate the cracks, ridges, and uneven surface of tree bark. Wet-on-wet and a flood of water spread smoothly and tend to reduce visible texture, while a flat light wash produces an even tone rather than a rough surface. Thus, B is the most suitable technique.