Which of the following best describes a firewall's functionality?

A firewall's primary functionality is to filter network traffic based on predefined security rules. It acts as a barrier between a trusted internal network and untrusted external networks, such as the internet. By inspecting incoming and outgoing traffic, a firewall can determine whether to allow or block specific data packets based on criteria such as source and destination IP addresses, protocols, and port numbers. This selective filtering helps prevent unauthorized access and protects the network from potential threats, such as malware and intrusions.

In contrast, the other options describe functionalities that are not directly related to a firewall's purpose. For instance, while secure data storage pertains to data protection strategies, it is not a feature of a firewall. Similarly, automatic software updates are managed by system management tools or update services, not by a firewall. Lastly, email encryption is a specific security measure that focuses on protecting the contents of emails, which falls outside the primary monitoring and filtering functions of a firewall.