The correct answer is D. subgrade.
The subgrade is the bottom most layer of pavement. It is the natural soil or compacted fill that supports the pavement structure. The subgrade must be strong enough to support the weight of the pavement and traffic without deforming.
The wearing course is the top most layer of pavement. It is the layer that is in direct contact with traffic. The wearing course must be durable enough to withstand the wear and tear of traffic.
The base course is the layer of pavement that is between the subgrade and the wearing course. It is designed to distribute the weight of the pavement and traffic over the subgrade. The base course must be strong enough to support the weight of the pavement and traffic without deforming.
The sub-base course is a layer of material that is placed between the base course and the subgrade. It is designed to improve the drainage and stability of the subgrade. The sub-base course is not always required, but it is often used in areas with poor subgrade conditions.