The correct answer is C. clay.
Plasticity index is a measure of the range of water contents over which a soil will deform continuously and plastically without cracking. It is calculated as the difference between the liquid limit and the plastic limit.
The liquid limit is the water content at which a soil will flow under a standard shear stress. The plastic limit is the water content at which a soil will just crumble when rolled into a thread 1/8 inch in diameter.
Clay has a high plasticity index because it is a fine-grained soil with a high percentage of clay minerals. Clay minerals are plate-shaped particles that have a strong attraction for water. This attraction causes clay to hold a lot of water, which makes it very plastic.
Sand, silt, and gravel are all coarse-grained soils with a low percentage of clay minerals. They do not hold as much water as clay, so they have a low plasticity index.
In conclusion, clay has a high plasticity index because it is a fine-grained soil with a high percentage of clay minerals. Clay minerals are plate-shaped particles that have a strong attraction for water. This attraction causes clay to hold a lot of water, which makes it very plastic.