What is the primary function of a firewall in network security?

The primary function of a firewall in network security is to monitor and control incoming and outgoing network traffic based on predetermined security rules. Firewalls act as a barrier between the trusted internal network and untrusted external networks, such as the internet. They analyze data packets that are transmitted and can permit or deny access based on defined security policies. This functionality is crucial in preventing unauthorized access, protecting sensitive information, and mitigating potential threats from malicious attacks.

While other options mention important aspects of network security, they do not encapsulate the fundamental role of a firewall. A physical barrier is not relevant in the context of how firewalls operate on a data level; firewalls do not provide data backup, which is a separate function in data management; and while encryption is important for securing communications, it is not a primary function of a firewall. Therefore, the answer correctly identifies the essence of what a firewall is designed to do in the realm of network security.