The correct answer is D. All of the above.
Corrugations, fins, and tubes are all methods of increasing the heat dissipating capability of transformers. Corrugations are grooves or ridges that are cut into the surface of the transformer. This increases the surface area of the transformer, which allows it to dissipate heat more effectively. Fins are thin, flat pieces of metal that are attached to the surface of the transformer. They increase the surface area of the transformer and also help to direct the flow of air over the transformer, which helps to cool it. Tubes are hollow cylinders that are filled with a coolant, such as oil or water. The coolant absorbs heat from the transformer and carries it away to a heat sink, where it is dissipated.
Transformers of ratings higher than 30 kVA generate a lot of heat. This heat can cause the transformer to overheat and fail. By providing corrugations, fins, or tubes, the heat dissipating capability of the transformer can be increased, which helps to prevent overheating and failure.