Which of the following is NOT a core component of VMware vSphere?

In the context of VMware vSphere, each component serves a specific role in the virtualization ecosystem. ESXi, vCenter Server, and vSphere Client are all integral to vSphere's functionality.

ESXi is the hypervisor that runs on the physical hardware and allows for the creation and management of virtual machines. vCenter Server is the centralized management platform that allows administrators to manage multiple ESXi hosts and provides advanced features such as vMotion and DRS. The vSphere Client provides the interface through which users interact with vCenter Server to perform various management tasks.

Active Directory, while a critical component in many IT environments for identity management and authentication, is not a core component of VMware vSphere itself. Instead, it operates separately and may be integrated with vSphere for user authentication purposes. Thus, it does not fall into the category of components that are essential for vSphere to function as a virtualization platform.