Every Nutanix cluster that will host NDB-provisioned databases must first be registered with the NDB control plane. Registration provides NDB with the Prism Element endpoint, credentials, storage-container information, and network context required to create and manage database server VMs on that target. A single NDB deployment can manage multiple registered Nutanix clusters; deploying another NDB appliance for every cluster is unnecessary. Database HA on the original cluster does not make an additional cluster available, and a protection domain is a data-protection construct rather than the NDB onboarding mechanism. Nutanix’s cluster-registration documentation explicitly requires registering the target cluster before databases can be provisioned there. Therefore, option C is the required preparatory action. Nutanix cluster-registration documentation