The correct answer is: A. Using a guard ring
A guard ring is an electrical conductor that is placed around the insulator of a high-voltage line. It helps to prevent corona discharge, which can cause power loss and damage to the insulator.
Grading the insulator is a process of applying a thin layer of a different material to the surface of the insulator. This helps to reduce the surface resistance of the insulator and improve its ability to withstand high voltages.
Using a long cross arm is a way of increasing the distance between the conductors of a high-voltage line. This helps to reduce the corona discharge and improve the efficiency of the line.
However, the most effective way to improve string efficiency is to use a guard ring.