The correct answer is C. Ring main system.
A radial system is a type of electrical distribution system in which each customer is connected directly to the source of power. This type of system is the simplest and least expensive to install, but it is also the least reliable. If there is a problem with the main line, all of the customers connected to that line will lose power.
A tree system is a type of electrical distribution system in which customers are connected to the source of power through a series of branches. This type of system is more reliable than a radial system, because if there is a problem with one branch, the other branches will still have power. However, tree systems are more expensive to install than radial systems.
A ring main system is a type of electrical distribution system in which customers are connected to the source of power through a series of loops. This type of system is the most reliable, because if there is a problem with one part of the loop, the other parts of the loop will still have power. Ring main systems are also the most expensive to install.
In conclusion, the ring main system is the most reliable type of electrical distribution system.