The correct answer is option 'B' - facing sand.

Explanation:
Facing sand is a type of sand used in the foundry industry as the first layer of sand in the mould cavity. It is applied on the inner surface of the mould cavity to provide a smooth and clean finish to the castings. In order to achieve this, a small amount of carbonaceous material is sprinkled on the inner surface of the mould cavity before applying the facing sand.

Facing sand is primarily responsible for giving the final shape and surface finish to the castings. It is typically made from high-quality silica sand mixed with a small proportion of clay and binders. The carbonaceous material sprinkled on the mould cavity serves as a parting agent and helps in easy separation of the mould from the castings.

The carbonaceous material used in facing sand can be in the form of coal dust, graphite, or other similar materials. When this material is sprinkled on the mould cavity, it forms a thin layer between the mould and the molten metal. This layer prevents the molten metal from sticking to the mould, allowing for easy removal of the casting after solidification.

In addition to providing a smooth finish, facing sand also helps in reducing defects such as sand inclusions, metal penetration, and veining on the surface of the castings. The carbonaceous material acts as a barrier, preventing the molten metal from infiltrating into the sand and causing defects.

Overall, facing sand plays a crucial role in the foundry process by providing a smooth finish and reducing defects on the surface of the castings. The small amount of carbonaceous material sprinkled on the inner surface of the mould cavity helps in achieving these objectives.