Which component of VMware vSphere is primarily responsible for virtualization?

The correct choice is the component responsible for virtualization within VMware vSphere is ESXi. ESXi is the hypervisor that allows multiple virtual machines (VMs) to run on a single physical server. It abstracts the hardware resources and allocates them to the VMs, effectively enabling hardware virtualization.

In a virtualized environment, ESXi hosts the VMs and manages their lifecycle, resource allocation, and isolation. This is crucial because it directly interacts with the physical hardware, providing the necessary functionality for running multiple operating systems and applications concurrently on a single server.

The other components play significant roles but do not provide the core virtualization functionality. vCenter Server is designed for centralized management of multiple ESXi hosts and VMs, but it does not perform the actual virtualization. The vSphere Client is a user interface for managing the virtual environment, allowing users to configure and monitor resources. VMware Tools is a suite of utilities installed on guest VMs to enhance performance and manageability but does not handle the virtualization of the hardware itself.