Which system is NOT considered a common electric system in a four-stroke engine?

In a four-stroke engine, the common electric systems typically include the starting, charging, and ignition systems. The starting system is essential for cranking the engine, the charging system maintains the battery's charge and powers the electrical components while the engine is running, and the ignition system is crucial for generating the spark that ignites the air-fuel mixture.

The cooling system, however, is not categorized as an electric system. Instead, it operates through fluid dynamics to manage the engine temperature and prevent overheating. It involves components like radiators, water pumps, and hoses, which work together to dissipate heat from the engine, but does not rely on electrical operations or systems to function. Therefore, recognizing that the cooling system operates independently from the electrical framework clarifies why it is the correct answer in this context.