Which of the following is NOT part of the hierarchy of domains?

The concept of domain hierarchy is essential in understanding how networks and directory services are structured. In this context, a domain refers to a collection of objects (such as users and computers) that are managed under a common administration within a network. A workgroup, while a network grouping of computers, operates at a more decentralized level and is distinct from domain-based structures, generally lacking centralized administration or directory services.

A forest is a collection of one or more domains that share a common schema and global catalog, playing a vital role in Microsoft Active Directory environments. It represents the top level of a hierarchy involving trees and domains.

In this setting, the term "cluster" does not fit within the hierarchy of domains. Clustering relates to grouping servers or systems to improve performance and provide redundancy or load balancing, which is a different concept entirely from the organizational structure of domains and forests. Thus, while domains, workgroups, and forests are integral parts of domain hierarchy, the term cluster does not pertain to this hierarchy.