The Correct Answer Is C

When a transformer is subjected to overheating, the part of the transformer that is most susceptible to damage is the winding insulation.

This is because when the transformer is supplied with too much load, the insulation can become damaged before the copper winding or iron core. Overheating can be caused by overexcitation, which results in over-currents, or by supplying too much load.

In order to prevent damage from overheating, it is important to ensure that the transformer is not overloaded and that any potential causes of overexcitation are accounted for.