Cohesive soils are primarily composed of which material?

Cohesive soils are primarily composed of clay particles. Clay is notable for its fine-grained texture and its ability to retain water, which contributes to its cohesive properties. When water is present, clay particles can exhibit plasticity, allowing them to be molded or shaped without breaking apart. This cohesion is a significant factor in the behavior of soils, especially in terms of stability and the ability to carry loads.

In contrast, silt, while also fine-grained, does not provide the same level of cohesiveness as clay. Sand consists of larger particles and drains water more quickly, leading to less cohesiveness due to a lack of fine particles that can attract and hold water. Rock, being a solid, non-soil material, does not fit into the definition of cohesive soils at all. Therefore, clay is the correct answer as it directly represents the characteristics associated with cohesive soils.