Adding or removing an application directory partition in AD DS -- including custom partitions such as the one App1 used to store its configuration data -- is an operation that must be performed against, and requires the availability of, the domain controller holding the Domain Naming Master FSMO role for the forest. This is the single operations master responsible for maintaining the list of domains and application partitions in the forest and for coordinating additions or removals of directory partitions; if that role holder is offline or unreachable, attempts to add or remove an application partition will fail, even though normal object read/write operations against other partitions (users, groups, and so on) may continue to function against any available domain controller. According to the exhibit ' s role assignments, DC3 holds the Domain Naming Master role. The Schema master (DC1) governs schema extensions, the Infrastructure master (DC2) governs cross-domain object reference updates, and the PDC emulator/RID master (DC4) govern time synchronization, password/authentication emulation, and relative ID pool allocation respectively -- none of these roles are involved in application partition add/remove operations. Because the removal of the custom application partition specifically failed, the unavailable domain controller must be the Domain Naming Master role holder, DC3.