The correct answer is: D. Both B and C
A foundation is a structural element that transfers the weight of a building or other structure to the ground. The foundation soil is the soil that supports the foundation. The sub-grade is the layer of soil that is prepared to support the foundation.
The foundation soil must be able to support the weight of the structure without settling or cracking. The sub-grade must be compacted and free of debris to provide a stable base for the foundation.
If the foundation soil is not strong enough, the foundation may settle or crack. If the sub-grade is not compacted or is not free of debris, the foundation may not be stable.
Therefore, it is important to choose a foundation type that is appropriate for the soil conditions and to prepare the sub-grade properly before installing the foundation.