The minimum value of potential gradient in a cable occurs in

The correct answer is: A. insulation.

The potential gradient is the rate of change of electric potential with distance. In a cable, the electric potential is highest at the conductor and lowest at the outer sheath. The insulation is in between the conductor and the outer sheath, so the potential gradient is lowest in the insulation.

Option B is incorrect because the conductor has the highest potential gradient. Option C is incorrect because the outer sheath has the lowest potential gradient. Option D is incorrect because the potential gradient is not uniform all over the cable.