Explanation:

Dielectric stress is the electrical stress experienced by the insulating material used in the cable. In an insulated cable, the maximum and minimum dielectric stresses are located at the outer and inner surfaces of the insulation respectively. The ratio of maximum to minimum dielectric stress can be calculated as follows:

Ratio of maximum to minimum dielectric stress = Electric field at outer surface / Electric field at inner surface

Now, the electric field is inversely proportional to the distance from the source of the field. Therefore, the electric field at the outer surface is less compared to the electric field at the inner surface. Hence, the ratio of maximum to minimum dielectric stress is greater than 1.

Let the thickness of the insulation be t. Then, the core diameter d and overall diameter D can be related as follows:

D = d + 2t

The electric field at the inner surface of the insulation can be calculated as follows:

Einner = V/d

where V is the voltage applied to the cable.

The electric field at the outer surface of the insulation can be calculated as follows:

Eouter = V/D

Substituting the value of D in terms of d and t, we get:

Eouter = V/(d+2t)

Therefore, the ratio of maximum to minimum dielectric stress can be calculated as follows:

Ratio of maximum to minimum dielectric stress = Eouter/Einner

= (V/(d+2t)) / (V/d)

= (d+2t)/d

= 1 + (2t/d)

As t is much smaller compared to d, the term (2t/d) can be neglected. Hence, the ratio of maximum to minimum dielectric stress is approximately equal to (D/d)2.

Therefore, option 'C' is the correct answer.