Which of the following describes the elasticity of hair?

The elasticity of hair refers to its ability to stretch and then return to its original shape without breaking. This characteristic is crucial in determining how hair responds to styling techniques, such as brushing, curling, or setting. When hair is elastic, it can withstand manipulation without sustaining damage, making it more adaptable to various hairstyles and treatments.

In contrast, the ability to break under stress is indicative of the hair’s strength, which is not the same as elasticity. The thickness of each hair strand pertains to the hair's diameter or texture rather than its elasticity. Similarly, the level of shine on the surface is a measure of hair's health and moisture content, but it does not correlate with how the hair stretches and recovers. Therefore, the correct choice emphasizes the capacity of hair to maintain its form after being manipulated, which is key for anyone working in barbering and cosmetology.