Which statement is true about the Primary bus in the electrical system of the C172N?

The primary bus in the electrical system of the C172N is a critical component responsible for distributing electrical power to various systems in the aircraft. When the master switch is activated, the primary bus is energized and remains on for the entire duration of the flight, allowing essential electrical systems to operate. This enables the aircraft's electrical components, such as avionics, lights, and other necessary systems, to function as required throughout the flight.

The other options do not correctly represent the role of the primary bus. It is not limited to only being on during startup, as it remains active whenever the master switch is engaged. It does not solely power the alternator; rather, the alternator charges the battery and supplies power to the primary bus, among other connections. Lastly, while there may be an emergency bus in the system to provide backup power in specific scenarios, the primary bus itself is primarily intended for regular operational needs, not for emergency power. Therefore, the understanding of the primary bus's continuous operation when the master switch is activated is essential to grasp its function within the aircraft's electrical system.