Comprehensive and Detailed Explanation From Exact Extract:

End-of-life (EOL) systems no longer receive security patches, vendor support, or vulnerability updates. Because of this, they are highly susceptible to exploitation, especially if attackers can reach them over a network. When the system is business-critical and cannot be decommissioned, the most effective strategy is isolation, also known as network segmentation, air-gapping, or restrictive network zoning. Isolation removes direct exposure to external and internal threats by limiting communication paths to only essential systems and users.

According to the Security+ SY0-701 guidance, isolating legacy systems helps reduce the attack surface when patching is no longer possible. Monitoring (A) is useful for detection but does not prevent exploitation. Decommissioning (C) would be ideal but is not possible for business-critical systems, as stated in the question. Encryption (D) protects data confidentiality but does not stop an attacker from exploiting vulnerabilities in an unpatched OS or application.

Isolation is a recommended compensating control for legacy and unsupported systems in SY0-701’s Security Architecture & Resilience domain, which emphasizes micro-segmentation, firewalls, and restricted access to minimize risk when systems cannot be replaced or patched.