What system supplies air conditioning packs during flight?

The correct choice is based on the A320's operational design concerning its air conditioning packs. The air conditioning packs on the A320 are pneumatically supplied by the engines. This is achieved through the engines' bleed air system, which extracts compressed air from the engine compressors. This bleed air is then routed to the air conditioning packs, where it is cooled and conditioned before being distributed to the cockpit and cabin.

Understanding this system is crucial because it highlights the integration of different systems within the aircraft, where engine operations directly influence cabin climate control. The reliance on engine bleed air means that the air conditioning packs are dependent on engine performance and operation, making the management of engine bleed systems vital for maintaining passenger comfort during flight.

Other options do not accurately represent the primary means of supplying air to the conditioning packs during flight, as the A320 specifically utilizes bleed air from its engines to fulfill this function.