The correct answer is D. insulator.
A cable capacitor is a type of capacitor that is used to store electrical energy. It consists of two conductors that are separated by an insulator. The voltage gradient is the rate of change of voltage with respect to distance. In a cable capacitor, the voltage gradient is maximum at the surface of the insulator. This is because the electric field is strongest at the surface of the insulator.
The earth is not a conductor, so there is no voltage gradient at the surface of the earth. The conduction is the flow of electricity through a conductor. The sheath is the outer layer of a cable, and it is not a conductor. Therefore, the voltage gradient is not maximum at the surface of the earth, the conduction, or the sheath.
The insulator is a material that does not conduct electricity. The electric field is strongest at the surface of the insulator, so the voltage gradient is maximum at the surface of the insulator.