Which of the following is a key characteristic of regolith?

Regolith is a layer of loose material covering solid rock, and it is primarily composed of weathered rocks, minerals, soil, and other loose fragments. This key characteristic allows regolith to serve as an important layer for various geological and ecological processes. The inclusion of soil suggests that regolith can support plant life, thereby playing a significant role in the earth's biosphere.

While the other options touch on aspects related to geology and soil, they do not accurately capture the essence of regolith. For instance, regolith is not solely made up of solid rock, as it distinctly includes various loose materials; it can contain organic matter, particularly when soil components are present; and it is not always found below the water table, since its location can vary based on topography and hydrology. Therefore, the correct identification of regolith as a mixture of soil and loose fragments highlights its diverse composition and significance in the environment.