Introduction:
The attainment of maximum density of soil at full saturation is a critical aspect in civil engineering. It is important to understand the factors that contribute to this maximum density as it affects the stability and strength of foundations, embankments, and other structures built on the soil.

Explanation:
The correct answer is option B, which states that the attainment of maximum density of soil at full saturation is due to hydrostatic pressure. Let's delve into this further to understand why.

Hydrostatic Pressure:
Hydrostatic pressure is the pressure exerted by a fluid (in this case, water) at rest. When soil is fully saturated, the void spaces between soil particles are filled with water, creating a hydrostatic pressure within the soil mass. This pressure acts uniformly in all directions and contributes to the densification of the soil.

Density of Soil:
The density of soil is a measure of how tightly the soil particles are packed together. It is usually expressed in terms of bulk density or dry density. The bulk density is the ratio of the total mass of soil to the total volume, including void spaces. The dry density is the ratio of the mass of solids to the total volume, excluding void spaces. The maximum density of soil is achieved when the soil particles are packed closely together, resulting in minimal void spaces.

Effect of Hydrostatic Pressure:
When soil is fully saturated, the hydrostatic pressure exerted by the water fills the void spaces between the soil particles. This pressure helps to compress the soil particles closer together, reducing the void spaces and increasing the soil density. As a result, the maximum density of soil is achieved at full saturation due to the hydrostatic pressure.

Importance in Civil Engineering:
The attainment of maximum density of soil is crucial in civil engineering for several reasons:

1. Stability: A densely packed soil has better stability and can withstand the load imposed by structures such as buildings, roads, and bridges. It minimizes settlement and prevents the occurrence of differential settlement, which can lead to structural damage.

2. Strength: The strength of soil is directly related to its density. A denser soil has greater shear strength and can resist applied forces better. This is particularly important in retaining walls, slopes, and other earthworks.

3. Permeability: Soil density affects its permeability, which is the ability of water to flow through the soil. A more densely packed soil has lower permeability, reducing the risk of water seepage and potential damage to structures.

Conclusion:
In conclusion, the attainment of maximum density of soil at full saturation is primarily due to the hydrostatic pressure exerted by the water within the soil mass. This pressure helps to compress the soil particles closer together, reducing the void spaces and increasing the soil density. Understanding this concept is essential in civil engineering to ensure the stability, strength, and durability of structures built on soil.