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.