Which type of system involves creating an unchangeable core that remains usable but not re-configurable?

An immutable system refers to a design in which the core components cannot be altered after they are created, but can still be utilized as needed. This principle supports the concept of security by ensuring that once a system is deployed, its base configuration remains unchanged, thereby protecting it from unauthorized modifications and potential vulnerabilities that could arise from reconfiguration.

In this context, an immutable system is particularly valuable in environments requiring high security and stability, as it simplifies maintenance and can significantly reduce the attack surface. If any changes are necessary, instead of modifying the existing system, new instances are deployed, preserving the integrity of the original system.

The other options, while related to security and system management, do not correctly embody the concept of having an unchangeable core. An allow list is a security measure that specifies which entities are permitted access to a resource, while a block list identifies those that are denied access. Antivirus software is designed to detect and remove malware but does not inherently restrict system configurations. Thus, the definition and operational model of an immutable system clearly align with maintaining a stable, secure core that remains consistent over time, validating its relevance to the question.