The correct answer is A. Rewireable fuse.
A rewireable fuse is a type of fuse that can be reset after it has blown. It is the least expensive type of fuse and is often used in low voltage systems.
An isolator is a device that is used to disconnect a circuit from the power supply. It is not a type of fuse and does not provide overcurrent protection.
An oil circuit breaker is a type of circuit breaker that uses oil to extinguish arcing. It is more expensive than a rewireable fuse and is typically used in high voltage systems.
An air break circuit breaker is a type of circuit breaker that uses air to extinguish arcing. It is more expensive than a rewireable fuse but less expensive than an oil circuit breaker.
Therefore, the least expensive protection for overcurrent in a low voltage system is a rewireable fuse.