The best way to reduce the immediate risk associated with using an unsupported version of the software is to segregate the outdated software system from the main network. This will limit the exposure of the system to potential attacks and prevent it from compromising other systems on the network. Segregating the system will also reduce the impact of any security incidents that may occur on the system.

Monitoring network traffic attempting to reach the outdated software system (option C) is not the best way to reduce the risk, as it will not prevent or stop any attacks on the system. It will only provide visibility into the network activity and alert the auditee of any suspicious or malicious traffic.

Verifying all patches have been applied to the software system’s outdated version (option A) and closing all unused ports on the outdated software system (option B) are also not the best ways to reduce the risk, as they will not address the underlying issue of using an unsupported version of the software. Patches and ports may still have vulnerabilities that are not fixed by the vendor, and attackers may exploit them to gain access to the system.

Therefore, option D is the correct answer.