Which of the following is a key function of a firewall in network security?

A firewall's primary role in network security is to block unauthorized access to or from a private network. This is accomplished through monitoring and controlling incoming and outgoing network traffic based on an applied rule set. By establishing a barrier between the trusted internal network and untrusted external networks (such as the internet), a firewall helps protect sensitive data and resources from potential external threats, including intrusions and attacks.

Firewalls can be configured to allow or deny specific data packets based on various criteria, such as IP addresses, protocols, or ports. This functionality is crucial in maintaining the integrity and security of the network by preventing unauthorized users and malicious software from gaining access to the network's internal resources.

The other options highlight important functions related to network management but do not represent the core functionality of a firewall. Encrypting email communications is a task typically handled by encryption protocols rather than firewalls. Enhancing data backup processes pertains to data management software and strategies rather than security devices. Managing internet browsing speed is a matter of bandwidth allocation and network optimization, which falls outside the purview of a firewall's primary function.