The correct answer is option 'A' - Density Index is the term used to express relative compactness of cohesionless soil only.

Explanation:
Density Index is a measure of the relative compactness or denseness of a soil. It is commonly used to evaluate the degree of compaction in cohesionless soils, such as sands and gravels. The Density Index is denoted by the symbol "ID" and is expressed as a decimal or a percentage.

The Density Index is determined by comparing the maximum and minimum void ratios of a soil sample. Void ratio is the ratio of the volume of voids to the volume of solids in a soil. The maximum void ratio (e_max) is the void ratio at the loosest state of the soil, while the minimum void ratio (e_min) is the void ratio at the densest state of the soil.

The formula to calculate the Density Index is as follows:
ID = (e_max - e)/(e_max - e_min)

where:
ID = Density Index
e_max = Maximum void ratio
e = Current void ratio
e_min = Minimum void ratio

In cohesionless soils, the void ratio is directly related to the degree of compaction. As the soil becomes more compacted, the void ratio decreases. Therefore, a higher Density Index indicates a more compacted soil, while a lower Density Index indicates a less compacted soil.

The Density Index is a useful parameter in geotechnical engineering as it helps in evaluating the potential settlement and strength characteristics of cohesionless soils. It is also used to determine the optimum compaction effort required during construction activities.

However, it is important to note that the Density Index is not applicable to cohesive soils. Cohesive soils, such as clays, have different compaction characteristics due to their cohesive nature. The compactness of cohesive soils is typically evaluated using other parameters, such as the Atterberg limits or the compaction curve.

In summary, the Density Index is a term used to express the relative compactness of cohesionless soils. It is calculated based on the maximum and minimum void ratios of the soil and provides a measure of the degree of compaction. However, it is not applicable to cohesive soils, which require different evaluation methods.

This term is used for cohesionless soil only. This term is not applicable to cohesive soil because of uncertaintities in the laboratory determination of the voids ratio in the loosest state of the soil (emax).