Final answer:
A network log monitoring system is likely the tool used to identify multiple RDP sessions from external IP addresses, as it allows for review of logged network activities.
Step-by-step explanation:
The tool that was likely used to examine the traffic and identify several Remote Desktop Protocol (RDP) sessions initiated from different Internet Protocol (IP) addresses is a network log monitoring system. Such a system keeps track of all the network traffic and logs various events which can then be reviewed. In contrast, while a network intrusion detection system (NIDS) and a network intrusion prevention system (NIPS) can detect and potentially stop malicious activity, and a network protocol analyzer could be used to examine traffic in real time or from captured data, the context provided points to the use of a system designed to monitor and log network events.