What is Chuck doing if he observes Alice's password sent in plain text?

If Chuck observes Alice's password being sent in plain text, he is engaging in eavesdropping. Eavesdropping refers to the unauthorized interception of communications or data as it is transmitted. This can occur when sensitive information, like passwords, is not adequately protected through encryption, enabling an external party to capture and read the data being sent over a network.

In this context, eavesdropping is primarily concerned with passive observation of data transmissions without actively interfering with the communication process. This means that Chuck is simply listening in or capturing the data as it travels, making it a stealthy way to gain access to sensitive information.

The options related to other actions, such as man-in-the-middle attacks or packet sniffing, would involve more active manipulation or specific techniques to intercept the data, while denial of service pertains to disrupting service rather than capturing data. Therefore, eavesdropping is the most accurate description of Chuck's actions in this situation.