What software is required to create this cluster of ESXi hosts?

Your company is evaluating data center virtualization on a single ESXi host. You intend to add
additional hosts, and perhaps even create a cluster of ESXi hosts configured for High Availability.
What software is required to create this cluster of ESXi hosts?

A.
vCloud Connector

B.
Site Recovery Manager

C.
vCloud Director

D.
vCenter Server

Explanation:
Reference:http://searchvmware.techtarget.com/What-is-VMware-vCenter-Server