The battery master switch function is

• A. Regulates engine oil pressure
• B. Controls cabin temperature
• C. Provides cosmetic lighting
• D. Connects or disconnects the battery from the electrical system

Answer: (D)

The battery master switch connects or disconnects the battery from the aircraft’s electrical system. This lets you isolate the battery to prevent parasitic drain when parked and to safely perform maintenance, while turning it on allows the battery to power the main electrical bus (and the starter) during operation. It also helps prevent back-feeding from the generator when the system is off.

This function is not about engine oil pressure, cabin temperature control, or cosmetic lighting, so those options don’t fit the role of the master switch.