Michelle discovers that a number of systems throughout her organization are connecting to a changing set of remote systems on TCP port 6667. What is the most likely cause of this, if she believes the traffic is not legitimate and that the systems are infected with malware?

This question combines two pieces of knowledge: how botnet command-and-control works, and that IRC's default port is TCP 6667. Although this could be one of the other answers, the most likely answer given the information available is a botnet that uses Internet Relay Chat (IRC) as its command-and-control channel. 6667 is not a common alternate web traffic port, peer-to-peer network traffic is commonly done via HTTP or HTTPS in modern infections, and a remote access-Trojan is likely to behave differently and use another port as well.