A circuit breaker primarily serves to...

The main function of a circuit breaker is to protect electrical circuits from overload. When too much current flows through a circuit, it can cause overheating and potential fires. A circuit breaker is designed to automatically cut off the electrical supply when it detects an overload or a short circuit, thereby preventing damage to the electrical system and ensuring safety.

In contrast, options related to connecting circuits, increasing voltage, or reducing resistance do not accurately depict the primary role of a circuit breaker. Connecting circuits pertains more to switches or connectors, while increasing voltage relates to transformers and power supply devices. Reducing electrical resistance usually involves different electrical components aimed at optimizing current flow rather than protecting against faults. Thus, the primary purpose of a circuit breaker centers on preventing overload rather than altering the circuit conditions directly.