The correct answer is D. to arrest flow of moisture when outside air enters the transformer.
A breather is a device that is installed on the top of a transformer tank. It allows the escape of any gases that may build up inside the tank, while preventing the ingress of moisture. This is important because moisture can cause the transformer oil to degrade, which can lead to a number of problems, including a decrease in the transformer’s efficiency and an increase in the risk of fire.
Option A is incorrect because the breather does not provide oxygen inside the tank. The transformer oil provides the oxygen that the transformer’s windings need to operate.
Option B is incorrect because the breather does not cool the coils during reduced load. The transformer’s cooling system is responsible for cooling the coils.
Option C is incorrect because the breather does not cool the transformer oil. The transformer’s cooling system is responsible for cooling the transformer oil.