The correct answer is: A. to withstand the high voltage drop due to line surge produced by the shunting capacitance of the end turns.
The end turns of a transformer are the two ends of the windings that are connected to the power lines. When a transformer is turned on, there is a surge of current that flows through the end turns. This surge of current can cause a high voltage drop across the end turns, which can damage the insulation. To prevent this damage, the end turns are heavily insulated.
The shunting capacitance of the end turns is the capacitance between the end turns and the transformer case. This capacitance can cause a high voltage drop across the end turns when a transformer is turned on. The heavy insulation on the end turns helps to reduce this voltage drop.
Options B and C are incorrect because they do not explain why the end turns of a transformer are heavily insulated. Option D is incorrect because it is not a valid option.