Pack valves will automatically re-open 30 seconds after:

The understanding of A320 systems requires familiarity with how the environmental control system operates, particularly concerning pack valves. When the first engine starts, the pack valves are initially closed to avoid engine bleed air supply to the aircraft’s climate control system if the second engine start is delayed. However, to ensure passenger comfort and appropriate air conditioning in the cabin, these valves are designed to automatically re-open 30 seconds after the first engine's start. This automated response allows for the efficient management of air supply even when one engine is operational, facilitating a smooth transition for cabin systems that rely on bleed air.

The other options relate to actions that do not specifically trigger the 30-second re-opening of the pack valves as described in this context. For instance, placing the ENG MODE selector to NORM or switching the ENG MASTER switch to ON does not generate the conditions necessary for this automatic valve operation, and starting the APU does not have the same effects on pack valve behavior in relation to engine status.