A security administrator is investigating the compromise of a SCADA network that is not physically connected to any other network. Which of the following is the MOST likely cause of the compromise?

Outdated antivirus definitions describe a vulnerability, not an attack vector - they do not explain how the compromise initially occurred on an air-gapped network.

Insecure wireless would effectively break the air gap, making it a network connection rather than a scenario consistent with a network 'not physically connected to any other network.'

SCADA networks that are physically isolated (air-gapped) cannot be reached via internet-based attacks, remote exploits, or wireless intrusion from outside the facility. Infected USB drives are a well-documented attack vector for air-gapped systems - threat actors (and real-world incidents like Stuxnet) demonstrate that removable media can carry malware across the physical boundary and execute once plugged into a device on the isolated network.

SQL injection requires network connectivity to a database-facing application and cannot be executed against a system with no external network connections.