On later A320s and all A319s, the emergency generator operates:

The emergency generator on later A320 models and all A319s is designed to provide power under specific conditions to ensure continued operation of essential systems during flight. It operates until touchdown regardless of the landing gear position, which ensures that critical systems remain powered until the aircraft has safely landed.

This function is crucial for maintaining control and communication with ground services during the final phases of flight, as well as supporting systems that might be necessary for the safe landing and evacuation procedures if required.

The other options, while they relate to power management, do not accurately describe the operational parameters of the emergency generator. For instance, the emergency generator is not solely dependent on the unpowered status of DC buses, nor does it only activate under conditions of failure of the main generators and when the auxiliary power unit is engaged. Additionally, the operational capacity of the emergency generator is not limited to the condition of the batteries; its independence from other power sources during critical phases of flight is what makes it a vital part of the aircraft’s emergency power system.