Which of the following is considered fine-grained soil?

Fine-grained soil is characterized by its small particle size, typically less than 0.075 mm in diameter. This category includes both silty and clay soils.

Silty soil, which is composed of fine, smooth particles, holds moisture well but has lower plasticity than clay. Consequently, it tends to have better drainage properties compared to clay. Clay soil, on the other hand, consists of even finer particles and has a high plasticity and significant cohesion, making it very malleable when wet but often problematic due to its expansion and contraction with moisture changes.

Both types contribute to the overall classification of fine-grained soils, making the selection of both silty and clay soil the most accurate choice. Other options, such as gravel, contain much larger particles and are classified as coarse-grained soil, which sets them apart from the fine-grained category. Thus, the inclusion of both silty and clay soil in this response correctly identifies the two primary types of fine-grained soils.