The correct answer is A. sand.
The plasticity index is a measure of the range of water contents over which a soil will exhibit plastic behavior. 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 small shear stress. The plastic limit is the water content at which a soil will just barely hold its shape when rolled into a thread 1/8 inch in diameter.
A soil with a plasticity index of zero is a non-plastic soil. This means that the soil will not exhibit plastic behavior, even when it is wet. This is typically the case for sands, which are composed of relatively large particles that do not interact with each other in a way that allows them to form a continuous matrix.
Silts and clays, on the other hand, are composed of much smaller particles that can interact with each other to form a continuous matrix. This allows them to exhibit plastic behavior, and their plasticity index will be greater than zero.
Clayey silts are a mixture of sand, silt, and clay. The clay content will give the soil some plasticity, but the sand content will limit the range of water contents over which the soil will exhibit plastic behavior. This means that the plasticity index of a clayey silt will be less than the plasticity index of a clay, but greater than the plasticity index of a sand.