Which firewall type is typically used on internal networks where systems are mostly trusted?

The correct choice aligns with the purpose and functionality of an internal firewall. Internal firewalls are strategically positioned within an organization's network to provide an additional layer of security among trusted systems. These firewalls monitor and control incoming and outgoing traffic, allowing organizations to segregate different segments of their internal network.

The internal firewall typically enforces security policies based on the trust level of various users and devices. Since the systems within the internal network are mostly trusted, the firewall can be configured with less stringent rules compared to those enforced by external firewalls. This is because the internal network is usually protected from external threats by a perimeter device, facilitating a more efficient flow of traffic while still maintaining a level of security that prevents compromised internal systems from affecting others.

Other options do not fit the context as well. An external firewall focuses on protecting the network from outside threats, a public firewall generally applies to open access situations where users are not trusted, and a DMZ is a special zone used to separate and protect internal resources from external access, typically containing publicly accessible services. Each of these options serves a different purpose in network security architecture, but for environments with trusted internal networks, the internal firewall is the most appropriate choice.