The correct answer is A. 31.25 percent of copper.
A 2-wire D.C. distributor has two conductors, one for the positive voltage and one for the negative voltage. A 3-wire D.C. distributor has three conductors, one for the positive voltage, one for the negative voltage, and one for the ground. The ground conductor is used to provide a safe path for current to flow in the event of a fault.
The amount of copper used in a distributor is proportional to the square of the voltage. This means that a 3-wire distributor with the same maximum voltage to earth as a 2-wire distributor will use only 31.25% of the copper.
Option B is incorrect because it is the amount of copper used in a 3-wire distributor with twice the maximum voltage to earth. Option C is incorrect because it is the amount of copper used in a 2-wire distributor with twice the maximum voltage to earth. Option D is incorrect because it is the amount of copper used in a 3-wire distributor with four times the maximum voltage to earth.