A systems administrator notices that one of the systems critical for processing customer transactions is...

To enhance security for a system running an end-of-life operating system, placing the system in an isolated VLAN is the most effective approach. By isolating the system from the rest of the network, you can limit its exposure to potential threats while maintaining its functionality. This segmentation helps protect the rest of the network from any vulnerabilities in the outdated system.

Installing HIDS (Host-based Intrusion Detection System) can help detect intrusions but won't mitigate the risks posed by an unsupported OS.

Decommissioning may not be feasible if the system is critical.

Encrypting the system's hard drive protects data at rest but doesn't address vulnerabilities from an outdated OS​​​.