The correct answer is: A. Pile formation.
A pile foundation is a type of foundation that transfers the load of a structure to a stronger underlying layer of soil or rock. Piles are long, slender columns that are driven into the ground, and they can be made of wood, steel, or concrete.
In a situation where a thick layer of fairly firm clay over a deep layer of soft clay is encountered, a pile foundation is the most suitable type of foundation. This is because the piles will transfer the load of the structure to the firm clay layer, which will provide a more stable foundation than the soft clay layer.
The other options are not suitable for this situation. A raft foundation is a type of foundation that spreads the load of a structure over a large area. This type of foundation is not suitable for a situation where there is a thick layer of fairly firm clay over a deep layer of soft clay, because the raft foundation would not be able to provide enough support for the structure. A grillage foundation is a type of foundation that consists of a series of closely spaced beams that are supported by piles. This type of foundation is also not suitable for a situation where there is a thick layer of fairly firm clay over a deep layer of soft clay, because the grillage foundation would not be able to provide enough support for the structure.