The correct answer is: A. Ring bus-bar scheme.
A ring bus-bar scheme is a type of electrical distribution system that uses a single bus bar that is looped back on itself. This type of system is relatively simple and inexpensive to install, and it offers good fault tolerance. However, it can be difficult to expand or modify a ring bus-bar system once it is in place.
A single bus-bar scheme is a type of electrical distribution system that uses a single bus bar that is not looped back on itself. This type of system is less expensive to install than a ring bus-bar system, but it is not as fault tolerant. If a fault occurs on a single bus-bar system, the entire system will be shut down.
A breaker and a half scheme is a type of electrical distribution system that uses two bus bars, each of which is fed by a separate circuit breaker. This type of system is more expensive to install than a single bus-bar system, but it is more fault tolerant. If a fault occurs on one bus bar, the other bus bar can continue to provide power to the system.
A main and transfer scheme is a type of electrical distribution system that uses two bus bars, one of which is the main bus bar and the other of which is the transfer bus bar. The main bus bar is normally in use, but if a fault occurs on the main bus bar, the transfer bus bar can be switched in to provide power to the system. This type of system is the most expensive to install, but it is the most fault tolerant.
In conclusion, the ring bus-bar scheme is the type of bus-bar scheme that has the lowest cost.