Dielectric strength of a medium is usually expressed in

J/mm
C/m2
kV/mm
N/mm

The correct answer is C. kV/mm.

Dielectric strength is the maximum electric field strength that a material can withstand without breaking down and becoming conductive. It is usually expressed in kilovolts per millimeter (kV/mm).

Option A, J/mm, is the unit of energy per unit volume. It is not used to express dielectric strength.

Option B, C/m2, is the unit of charge per unit area. It is not used to express dielectric strength.

Option D, N/mm, is the unit of force per unit length. It is not used to express dielectric strength.