Final answer:
Clay has the greatest chance of shifting during excavation due to its fine particles and its ability to retain water, which can cause expansion and contraction. Option C is correct.
Step-by-step explanation:
The type of soil that has the greatest chance of shifting during excavation is clay. Clay consists of very fine particles and silt that give the material its plasticity, particularly when water is added, making it a cohesive product. Due to its fine particles and its ability to retain water, clay can expand and contract, which can lead to shifting during the excavation process. This shifting can pose challenges for laying a stable foundation, as the soil may move in response to changes in moisture and pressure.
On the other hand, gravel and sand have larger, coarser particles which allow for better drainage and less water retention, thereby reducing the potential for soil shifting. Topsoil is generally a layer rich in organic material and not typically the layer in which foundations are set. However, all types of soil can potentially shift if they are not properly managed or if unusual weather conditions are present, so careful consideration and soil testing is necessary before any excavation.