The correct answer is D. Its molecular structure will get changed.
When a voltage more than the breakdown voltage is applied on an insulating medium, the electrons in the medium will gain enough energy to break free from their atoms. This will cause the medium to become conductive, and it may also cause the medium to get punctured or cracked. The molecular structure of the medium will also be changed, as the electrons will be rearranged.
Option A is incorrect because an insulating medium will not become magnetic when a voltage is applied to it. Option B is incorrect because an insulating medium will not melt when a voltage is applied to it. Option C is incorrect because an insulating medium may get punctured or cracked when a voltage more than the breakdown voltage is applied to it, but this is not always the case.