The correct answer is option D) Subgrade.

The pavement structure is composed of multiple layers, each with its own purpose and function. Starting from the bottom, the bottommost layer of pavement is known as the subgrade. Let's explore this layer in detail.

Subgrade:
The subgrade is the natural soil or compacted fill material on which the pavement is constructed. It is the foundation or base layer of the pavement structure and plays a crucial role in supporting the loads imposed by vehicles and distributing them to the underlying soil.

Functions of Subgrade:
1. Load Support: The primary function of the subgrade is to provide a stable and uniform support for the pavement layers above. It must be able to withstand the loads from vehicles without excessive deformation or settlement.

2. Load Distribution: The subgrade helps distribute the applied loads from the pavement layers to the underlying soil. It spreads the load over a larger area to prevent localized failures and excessive stresses.

3. Drainage: Proper subgrade drainage is essential to prevent the accumulation of water, which can weaken the soil and decrease its load-bearing capacity. Adequate drainage helps maintain the stability of the subgrade and the entire pavement structure.

4. Frost Protection: In regions with freezing temperatures, the subgrade must be designed to resist frost heave. Frost heave occurs when water in the soil freezes and expands, causing upward movement and potential damage to the pavement.

Subgrade Preparation:
To ensure a durable and long-lasting pavement, proper subgrade preparation is crucial. It involves the following steps:

1. Excavation: Any unsuitable or weak soil is removed, and the subgrade is brought to the desired elevation. Excavation may also include the removal of vegetation, organic materials, or debris that could affect the performance of the subgrade.

2. Compaction: The subgrade is compacted to increase its density and strength. This is typically done using compaction equipment, such as rollers or vibratory plates, to achieve the desired level of compaction and stability.

3. Moisture Control: Moisture content in the subgrade should be controlled to maintain its stability. Excessive moisture can lead to softening and reduced load-bearing capacity. Proper drainage systems should be installed to manage water infiltration and control the moisture content of the subgrade.

Conclusion:
The subgrade is the bottommost layer of the pavement structure and serves as the foundation for the entire pavement system. It provides load support, load distribution, drainage, and frost protection. Proper subgrade preparation is essential to ensure the performance and longevity of the pavement.