Which of the following best describes the function of a firewall?

A firewall functions primarily to block unauthorized access to or from a private network. It acts as a barrier between a trusted internal network and untrusted external networks, such as the Internet. By monitoring and controlling incoming and outgoing network traffic based on predetermined security rules, firewalls help to protect against threats such as intrusions, attacks, and unauthorized data access. This protective role is crucial in maintaining the security and integrity of the system it safeguards.

The other options describe different functions that are not related to the core purpose of a firewall. For example, encryption is about securing data by converting it into a form that cannot be easily understood by unauthorized users, which is not what a firewall does. Enhancing internet speed is not a function of firewalls; rather, they can occasionally introduce latency due to the processing of network traffic. Finally, restoring deleted files is related to data recovery or backup solutions, and is outside the scope and capability of a firewall's protective function.