How many slide packs are there for the A320, A321, and A321NX?

For the A320, A321, and A321NX aircraft, the correct number of slide packs is one at each door. This setup is designed to ensure that in an emergency evacuation situation, each door is equipped with a single slide that serves as both an evacuation slide and a flotation device for over-water landings. Having one slide pack per door allows for a streamlined and efficient evacuation process, catering to the need for quick and orderly egress from the aircraft while maintaining a balance with the overall weight and system efficiency of the aircraft.

The other options imply that there are multiple slides at each door, which would unnecessarily complicate the design and evacuation procedure without providing additional safety benefits. Thus, the configuration of one slide at each door is both practical and effective for the aircraft models mentioned.