The correct answer is: A. 1.5 MVA
Natural air cooling is a type of transformer cooling that uses the natural circulation of air to remove heat from the transformer. It is the simplest and most economical type of transformer cooling, but it is also the least efficient. Natural air cooling is generally restricted for transformers up to 1.5 MVA.
Transformers with higher ratings require more cooling than can be provided by natural air cooling. These transformers are typically cooled by forced air cooling or by oil cooling.
Forced air cooling uses fans to circulate air over the transformer. This type of cooling is more efficient than natural air cooling, but it is also more expensive.
Oil cooling uses oil to circulate heat away from the transformer. This type of cooling is the most efficient, but it is also the most expensive.