Liquidity index is the correct answer because it represents the ratio between the water content and the plastic limit of a soil mass. It is a measure of the consistency or liquidity of the soil.

Explanation:

Liquidity index is an important parameter used in geotechnical engineering to classify the behavior of soils. It is defined as the ratio of the difference between the water content and the plastic limit to the plasticity index of the soil.

The plastic limit is the water content at which a soil transitions from a plastic to a semisolid state. It is determined by a standard test known as the Casagrande method. The plasticity index, on the other hand, is the numerical difference between the liquid limit and the plastic limit of the soil. It represents the range of water content within which the soil can exhibit plastic behavior.

The liquidity index is calculated using the following formula:
Liquidity Index (LI) = (Water Content - Plastic Limit) / Plasticity Index

The liquidity index provides information about the consistency and behavior of the soil. It is a measure of the soil's liquidity, which refers to its ability to flow under its own weight. Soils with a high liquidity index are more liquid-like and tend to have higher water contents, while soils with a low liquidity index are more solid-like and have lower water contents.

The liquidity index can be used to classify soils into different consistency states, such as liquid, plastic, or semisolid. For example, a liquidity index less than 0 indicates a solid-like state, while a liquidity index greater than 0 indicates a liquid-like state.

In summary, the ratio of water content minus plastic limit to plasticity index is known as the liquidity index. It is an important parameter used in geotechnical engineering to classify the consistency and behavior of soils.