In the scenario where vulnerabilities exist in an out-of-support proprietary system, conventional mitigation strategies like patching are not feasible. Therefore, compensating controls are the appropriate form of mitigation to suggest. Compensating controls refer to security measures that are put in place to satisfy the requirement for a security control that is deemed too difficult or impossible to implement at the present time. By framing the vulnerability in terms of inhibitors to remediation and suggesting compensating controls, the cybersecurity analyst shows an understanding of the constraints of proprietary systems. Other answers are less appropriate because they either ignore the proprietary and unsupported context of the system or suggest an unfit remediation strategy.