Which is the correct order for upgrading vSphere components?

A.
vCenter Server, ESXi hosts, VMware Tools, Virtual Machine hardware

B.
VMware Tools, Virtual Machine hardware, ESXi hosts, vCenter Server

C.
vCenter Server, Virtual Machine hardware, VMware tools, ESXi hosts

D.
ESXi hosts, vCenter Server, Virtual Machine hardware, VMware Tools

Explanation:
The correct way to upgrade vSphere is to start with vCenter Server. Then comes ESXi hosts. After that you
should upgrade VMware tools and finally virtual machine hardware.
https://pubs.vmware.com/vsphere-50/index.jsp?topic=%2Fcom.vmware.vsphere.upgrade.doc_50%
2FGUID-A45CBEE5-C4D2-484E-A699-A5A577244DE0.html