What system is considered a Mandatory Access Control (MAC) system?

The correct choice is SELinux, which is a well-known example of a Mandatory Access Control (MAC) system. MAC systems enforce access controls based on predefined policies, rather than user permissions or individual decisions, which is typical in Discretionary Access Control (DAC) systems.

SELinux operates by applying security policies that dictate how processes and users interact with one another and with the files on the system. This enhances security by enforcing that a user or process can only access resources for which they have explicit permission set by the policies. The configuration and policies are generally centrally controlled, which strengthens security by minimizing the potential for human error in access decisions.

In contrast, AppArmor is often described as a more user-friendly, profile-based system for device control and might not enforce MAC in the same defined manner as SELinux. Yum is a package management tool for Linux, and iptables is used for setting up, maintaining, and inspecting the tables of IP packet filter rules in the Linux kernel, neither of which relates to access control mechanisms.