Explanation:
In soil mechanics, the plastic limit (PL) and liquid limit (LL) are two important properties used to determine the plasticity index (PI) of a soil. The plasticity index is a measure of the range of water content within which a soil exhibits plastic behavior.

Plastic Limit (PL):
The plastic limit is the water content at which a soil changes from a plastic state to a semi-solid state. It represents the minimum water content at which the soil can be rolled into a thread of 3 mm diameter without breaking. Plastic limit is determined by conducting the plasticity test on a soil sample.

Liquid Limit (LL):
The liquid limit is the water content at which a soil changes from a plastic state to a liquid state. It represents the water content at which the soil begins to flow and lose its plasticity. Liquid limit is determined by conducting the Casagrande's liquid limit test on a soil sample.

Plasticity Index (PI):
The plasticity index is a numerical value that represents the range of water content within which a soil exhibits plastic behavior. It is calculated by subtracting the plastic limit (PL) from the liquid limit (LL).

Interpretation of PI:
- If the plastic limit is greater than the liquid limit (PL > LL), it indicates that the soil has a low plasticity. This means that the soil has a narrow range of water content within which it exhibits plastic behavior.
- When the plastic limit is greater than the liquid limit, the plasticity index is reported as zero (PI = 0). This is because there is no range of water content where the soil exhibits plastic behavior. The soil is considered non-plastic (NP) in this case.
- On the other hand, if the liquid limit is greater than the plastic limit (LL > PL), it indicates that the soil has a high plasticity. This means that the soil has a wider range of water content within which it exhibits plastic behavior. The plasticity index will be a positive value in this case.

Therefore, the correct answer is option 'B' - Zero (0).