Implementing network segmentation and limiting the legacy system's connectivity to essential services is the correct answer. This approach reduces the risk of attacks from both internal and external threat vectors, as it would prevent the potentially compromised system from affecting unrelated parts of the network. While increasing security monitoring is a useful tactic, it does not directly mitigate the exposure of the legacy system to threats. Conducting regular security audits on the system is a good practice, but it does not provide real-time protection against threats. Encouraging the use of strong passwords is important but does not address the specific risks associated with an unsupported operating system.