Final answer:
Clay sediments are the most unstable and at high risk for mass movement, especially when waterlogged or under environmental stresses such as earthquakes or over-steepening due to erosion.
Step-by-step explanation:
The question concerns which types of sediments are unstable and at a high risk for mass movement. Among the options provided, sand, gravel, clay, and silt, clay is the most prone to instability under certain conditions. Clay particles are very small and tend to stick together due to their platy nature. When water is added, clay becomes a cohesive and plastic material. This can increase the risk of a mass movement, particularly when clay is part of a slope that becomes saturated with water from heavy rains or snowmelt. In contrast, the larger particles of sand and gravel typically have better drainage and are less likely to move. Silt, while smaller in size than sand and gravel, is also less prone to mass movement than clay. Additionally, areas with clay can be more susceptible if they are over-steepened by erosion from rivers, glaciers, or ocean waves, or stressed by factors such as earthquakes or volcanic eruptions.