What happens to the Essential Bus when the battery master switch is turned on?

When the battery master switch is turned on, the Essential Bus is activated, allowing key systems essential for the operation and safety of the aircraft to receive power. The Essential Bus provides electrical power to critical components necessary for flight operations, such as navigation and communication instruments. This action ensures that the pilot can monitor and control vital systems immediately after the aircraft's electrical system is energized.

The activation of the Essential Bus is crucial as it prepares the aircraft for subsequent steps in the start-up procedure. While other options listed might involve separate processes, the primary focus with the battery master switch is to enable the Essential Bus, thereby powering critical electrical components essential for the aircraft's readiness.