The correct answer is option 'A': Subgrade.

Subgrade refers to the layer that is constructed above an embankment. It plays a crucial role in providing a stable foundation for the pavement or structure that will be built on top of it. Let's delve into the details of what a subgrade is and its significance in the construction process.

Subgrade: The Foundation Layer
The subgrade is the natural or prepared surface upon which the embankment is constructed. It acts as the foundation layer for the subsequent layers of the pavement or structure. The quality and stability of the subgrade directly impact the performance and longevity of the overall construction.

Functions of Subgrade
The subgrade performs several important functions in the construction process:

1. Load Distribution: The primary function of the subgrade is to evenly distribute the load from the embankment and the traffic above it. It should have sufficient bearing capacity to support the imposed loads without excessive settlement.

2. Stability: The subgrade must provide a stable platform for the pavement or structure. It should resist deformation and settlement caused by the applied loads, ensuring that the surface remains even and level.

3. Drainage: An essential aspect of subgrade design is ensuring proper drainage. It should be adequately designed to drain away water, preventing the accumulation of moisture that can weaken the subgrade and lead to instability.

4. Frost Protection: In areas with freezing temperatures, the subgrade needs to be designed to resist the detrimental effects of frost heave. Adequate measures, such as proper drainage and insulation, should be implemented to mitigate frost-related issues.

Construction of Subgrade
The construction process of the subgrade involves the following steps:

1. Site Preparation: The area where the subgrade will be constructed is cleared of any vegetation, debris, or other obstructions. The underlying soil is then compacted to ensure a stable foundation.

2. Subgrade Material: Suitable materials, such as granular or cohesive soils, are added to the prepared area to achieve the desired subgrade thickness. These materials are then properly compacted to improve their strength and load-bearing capacity.

3. Compaction: Compaction is a critical process in subgrade construction. It involves the use of heavy machinery, such as rollers or compactors, to compress the soil particles and eliminate air voids. This enhances the stability and load-bearing capacity of the subgrade.

4. Testing and Quality Control: Various tests, such as plate load tests or CBR (California Bearing Ratio) tests, are conducted to assess the quality and strength of the subgrade. This ensures that it meets the required specifications and can adequately support the subsequent layers.

In conclusion, the subgrade is the layer constructed above an embankment, serving as the foundation for the pavement or structure. It plays a crucial role in load distribution, stability, drainage, and frost protection. Proper construction techniques, including site preparation, suitable materials, compaction, and quality control, are essential to ensure a stable and durable subgrade.