Which characteristic of bamboo brushes contributes to their flexibility in art applications?

The flexibility of bamboo brushes in art applications is primarily influenced by the size of the brush. Larger brushes tend to have more bristles, which can create a softer and more flexible application when painting or inking. This flexibility allows artists to achieve different effects, such as varying line widths and brush strokes, which are essential for techniques like calligraphy or watercolors.

Other characteristics like the color of the bristles or the angle at which the brush is held can affect the final appearance and technique but do not directly contribute to the inherent flexibility of the brush itself. Similarly, the type of handle might influence comfort and control during use, but it does not play a significant role in the flexibility that is crucial for diverse art applications.