Elizabeth finds that a number of systems throughout her organization are connecting to a changing set of remote systems on TCP port 6667. Which of the following is the most likely cause of this, if she thinks the traffic is NOT legitimate?

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.