The correct answer is C. 500 kVA.
Natural oil cooling is a type of transformer cooling that relies on the natural circulation of oil to remove heat from the transformer. This type of cooling is typically used for transformers with a rating of up to 500 kVA.
Natural oil cooling is a simple and inexpensive type of cooling, but it is not as efficient as other types of cooling, such as forced oil cooling. Natural oil cooling is also not as effective in removing heat from transformers that are operating at high temperatures.
When natural oil cooling is used, the transformer oil is heated by the transformer windings. The hot oil rises to the top of the transformer and flows through a radiator, where it is cooled by the air. The cooled oil then flows back down to the bottom of the transformer and repeats the cycle.
The efficiency of natural oil cooling depends on the temperature difference between the transformer windings and the ambient air. The greater the temperature difference, the more efficient the natural oil cooling will be.
Natural oil cooling is a reliable and safe type of cooling, but it is important to monitor the transformer oil temperature to ensure that it does not exceed the safe operating limits.
The other options are incorrect because they are not the typical rating for transformers that use natural oil cooling.