Final answer:
The best explanation for a worm infiltrating an air-gapped nuclear plant system is the introduction of a malicious USB by an unsuspecting employee. This method bypasses the conventional network security systems in place as the isolated network cannot be accessed remotely.
Step-by-step explanation:
The question you asked relates to a cyber security incident where a nuclear plant's network, which was air-gapped, was compromised by a worm. The most plausible explanation for how this could occur is A. A malicious USB was introduced by an unsuspecting employee. Since air-gapped networks are isolated from the Internet, the introduction of malware through a network intrusion is not possible. Instead, the worm had to be physically introduced into the network, most likely through infected external media such as a USB drive. This scenario is reminiscent of the famous Stuxnet worm, which targeted industrial systems and was spread through removable drives. It bypasses network security measures that would typically protect against remote cyberattacks because air-gapped systems are isolated from external networks.
Although the possibility of outdated ICS firmware or a Remote Access Trojan (RAT) on a local machine could contribute to an attack's success, these factors would not initiate an attack on an air-gapped system; rather, they could exacerbate the problem once the malware has been introduced into the system. Similarly, the connection of the HVAC system to the maintenance vendor does not directly pertain to the delivery mechanism of the worm, unless it was explicitly used to bridge the air gap, which seems less likely in this context.