In case of AC bus 1 failure, the AC ESS bus can be transferred to AC bus 2 by:

The ability of the AC ESS bus to be automatically transferred to AC bus 2 in the event of an AC bus 1 failure is a critical feature of the A320 aircraft's electrical system designed for safety and reliability. In the event that AC bus 1 is lost, the electrical architecture of the A320 ensures that the AC ESS bus can switch over to AC bus 2 without requiring any manual intervention.

This automatic switching capability allows for continued power supply to essential systems, which is particularly important during emergency situations. The design minimizes the need for pilot action, allowing the crew to focus on other critical aspects of aircraft operation during a failure. This redundancy in the electrical system is testament to the reliability engineered into the aircraft to maintain safe flight even when certain components experience failure.

The other options involve actions that are not required for the automatic function to occur or do not contribute to the process of transferring the AC ESS bus to AC bus 2 during an AC bus 1 failure. Hence, option B correctly highlights the design feature that provides safety and operational continuity.