The correct answer is A. Conservator.
A conservator is a tank that is connected to the transformer’s main tank by a pipe. The conservator tank is filled with oil, and the pipe allows the oil to flow between the two tanks. As the temperature changes, the volume of the oil in the transformer tank changes. The oil flows into or out of the conservator tank to keep the oil level in the transformer tank constant.
A breather is a device that allows air to enter and exit the transformer tank. The breather prevents the build-up of pressure or vacuum in the tank.
Bushings are insulators that connect the transformer’s windings to the outside world. They prevent current from flowing through the transformer’s housing.
A Buchholz relay is a device that detects and protects against internal faults in the transformer. It does not have anything to do with the changes in volume of the transformer cooling oil.