The correct answer is: D. to take care of the expansion and contraction of transformer oil due to variation of temperature of surroundings.
A conservator is a tank that is connected to the transformer oil tank by a pipe. The conservator tank is filled with air, and the pipe between the two tanks is filled with a breather. The breather allows air to flow into the conservator tank when the transformer oil expands, and it allows air to flow out of the conservator tank when the transformer oil contracts. This prevents the transformer oil from overflowing the transformer oil tank.
The other options are incorrect. Option A is incorrect because the conservator does not protect against internal faults. Option B is incorrect because the conservator does not reduce copper or core losses. Option C is incorrect because the conservator does not cool the transformer oil.