Compaction of soil is a process in which the soil particles are rearranged and air voids are reduced in order to increase the dry density of the soil. The dry density of the compacted soil is a key parameter used to assess the quality and performance of the soil for various engineering applications. Therefore, the measurement of compaction is commonly expressed in terms of dry density.

Here's the detailed explanation of why the correct answer is option 'A' (dry density):

1. Importance of compaction:
Compaction is an essential process in soil engineering as it improves the soil's load-bearing capacity, reduces settlement, and increases stability. It is widely employed in the construction of roads, embankments, foundations, and other structures.

2. Dry density:
Dry density is defined as the mass of solid soil particles per unit volume of the compacted soil. It is usually expressed in grams per cubic centimeter (g/cm³) or kilograms per cubic meter (kg/m³). Dry density is a measure of the soil's compactness and is influenced by factors such as particle size, moisture content, compaction effort, and soil type.

3. Measurement of dry density:
The dry density of compacted soil is determined using a laboratory test called the Proctor compaction test. In this test, a soil sample is compacted using a standardized compaction energy and the dry density is calculated by dividing the mass of solid particles by the volume of the compacted soil.

4. Compacting energy:
The compaction energy applied during the Proctor test can be either Standard Proctor or Modified Proctor. The Standard Proctor test is commonly used for fine-grained soils, while the Modified Proctor test is used for coarse-grained soils. The compaction energy influences the maximum achievable dry density and optimum moisture content of the soil.

5. Relationship with other properties:
Dry density is related to various soil properties such as porosity, void ratio, and degree of saturation. It is inversely proportional to porosity and void ratio, meaning that as the dry density increases, the porosity and void ratio decrease. Additionally, the dry density affects other geotechnical properties like shear strength, permeability, and compressibility of the soil.

In conclusion, the compaction of soil is measured in terms of dry density because it provides a quantitative measure of the soil's compactness and is crucial for evaluating its engineering properties and performance.