The correct answer is D) using higher voltage and lower current. The reason this is the correct answer is that using higher voltage and lower current helps minimize line losses in a distribution circuit. This is because line losses are proportional to the square of the current flowing through the line, as described by the formula P = I^2 * R, where P is the power loss, I is the current, and R is the resistance of the line. By using higher voltage and lower current, the current is reduced, which in turn reduces the line losses. The other options are incorrect because they do not directly address the relationship between voltage, current, and line losses. Adding rubber insulation or increasing the number of thermal relays does not affect the line losses. Using higher current and lower voltage would actually increase the line losses.