The correct answer is C. non-plastic (NP).
The plasticity index (PI) is a measure of the range of water contents over which a soil will exhibit plastic properties. It is calculated by subtracting the liquid limit (LL) from the plastic limit (PL). If the PL is greater than the LL, then the soil is non-plastic and the PI is reported as NP.
A soil is said to be plastic when it has a certain range of water contents and can be molded and deformed without breaking apart. The LL is the water content at which a soil has just lost its liquid properties and begins to exhibit plastic properties. The PL is the water content at which a soil has just lost its plastic properties and begins to exhibit semi-solid properties.
If the PL is greater than the LL, then the soil is non-plastic. This means that the soil does not have a range of water contents over which it exhibits plastic properties. The soil will be either solid or liquid, depending on the water content.
Soils that are non-plastic are typically sandy or gravelly soils. They are not very cohesive and do not hold together well. They are also not very compressible and do not deform easily.
Non-plastic soils are often used in construction as fill material. They are also used in some types of landscaping.