Which method is effective in preventing eavesdropping on a network?

Encrypting data in transit is an effective method for preventing eavesdropping on a network because it transforms the data being transmitted into a secure format that can only be read by authorized parties. This means that even if the data is intercepted during transmission, it remains unreadable to unauthorized individuals, thereby protecting sensitive information from exposure.

Encryption uses algorithms to scramble the data, making it nearly impossible for anyone without the correct decryption key to interpret it. This process is crucial in protecting personal information, financial transactions, and confidential communications over networks, especially unsecured ones like public Wi-Fi.

The other methods offered, while useful for different aspects of network security, do not specifically address the threat of eavesdropping in the same way that encryption does. For example, digitally signing messages ensures message integrity and authenticity but doesn't conceal the content from interception. Using plain text messages exposes information to anyone who might be listening in, making it highly vulnerable to eavesdropping. Setting up firewalls helps protect a network from unauthorized access but does not prevent the interception of data that is already being sent over the network. Thus, encryption is the strongest line of defense against eavesdropping, making it the correct choice in this context.