Before you upgrade an ESX host what should you do?

A.
back up /proc, /mnt, /tmp, custom scripts and all VMFS datastores

B.
back up /home, /etc/init.d and /boot

C.
back up /etc/passwd, /etc/groups, /etc/shadow, /etc/gshadow, custom scripts and local VMFS

D.
back up /var/logs, /usr, /bin and the dmesg output

Explanation:
vSphere Upgrade Guide ESX, ESXi, vCenter Server, vSphere Client, page 71 `Before you upgrade an ESX host, back up the local VMFS file system. This ensures that you will not lose data during the upgrade.
Procedure
+ Back up the files in the /etc/passwd, /etc/groups, /etc/shadow, and /etc/gshadow directories. The /etc/shadow and /etc/gshadow files might not be present on all installations.
+ Back up any custom scripts.
+ Back up your .vmx files.
+ Back up local images, such as templates, exported virtual machines, and .iso files.’