Which description best fits 'contraction' in terms of soil behavior?

The concept of 'contraction' in terms of soil behavior refers to the tendency of soil to shrink or decrease in volume as it dries out or loses moisture. Therefore, the most fitting description is the one that highlights how soil changes significantly in response to moisture content. This behavior is particularly associated with expansive clays, which can swell when wet and contract when dry.

Understanding this principle is crucial for construction and civil engineering projects, as soil contraction can impact the stability and integrity of structures. The focus on changes with moisture content encapsulates the fundamental property of contraction, making it the most appropriate choice in this context.

Other options, while they may touch on various aspects of soil behavior, do not specifically address the defining characteristic of contraction related to moisture dynamics.