The correct answer is: B. 5 MVA
Transformers with a capacity of up to 5 MVA have a cooling radiator system with natural cooling. This means that the transformer does not require any external fans or pumps to circulate the cooling fluid. The cooling fluid is typically oil, which is circulated through the transformer by natural convection. This type of cooling system is relatively simple and inexpensive, but it is not as efficient as forced-air or forced-water cooling systems.
Transformers with a capacity of more than 5 MVA typically require forced-air or forced-water cooling systems. These systems are more efficient than natural cooling systems, but they are also more expensive. Forced-air cooling systems use fans to circulate air over the transformer’s cooling fins. Forced-water cooling systems use pumps to circulate water over the transformer’s cooling fins.
The type of cooling system that is used for a transformer depends on the transformer’s capacity, the ambient temperature, and the transformer’s application. For example, a transformer that is located in a hot environment may require a forced-air or forced-water cooling system, even if it has a capacity of less than 5 MVA.