How does a public firewall function compared to an external firewall?

A public firewall is designed to protect a network from untrusted sources while allowing communication with external networks, such as the internet. Its primary function is to monitor and control incoming and outgoing network traffic based on predetermined security rules. This makes option C correct, as a public firewall is typically implemented in situations where untrusted network systems, like potentially harmful traffic from the internet, are present.

The role of a public firewall is to establish a barrier between a trusted internal network and external networks that contain unknown or potentially malicious entities, allowing only certain types of traffic while blocking others to enhance security.

In contrast, the other options do not accurately describe the role of a public firewall. Trusted internal networks do not use public firewalls; instead, they might implement internal firewalls or different security measures. Public firewalls are designed to limit incoming connections rather than allow more, as their purpose is to protect against unauthorized access. Lastly, a controlled environment would imply that all traffic can be trusted, which does not align with the purpose of a public firewall; its role is to safeguard against untrusted traffic rather than operate in an inherently secure or controlled environment.