An air gap is the practice of physically isolating a secure network from any external or unsecured networks, effectively preventing any external communication or data leakage. It is the strongest method to prevent data exfiltration in sensitive environments.
Containerization (B) and virtualization (C) are technologies for isolating applications or systems logically but do not guarantee physical separation. Decentralization (D) distributes resources but doesn't prevent data leakage.
Air gaps are critical in highly secure environments and covered under Resilience and Physical Security in SY0-701#6:Chapter 9 CompTIA Security+ Study Guide#.