Pointing a VI Client directly to an ESX Server host rather than a VirtualCenter Server:

A.
is not recommended, even if VirtualCenter was not purchased.

B.
works identically, but any tasks issued are superceded by the VirtualCenter Server.

C.
is similar except that objects are at the host and not datacenter level, and enterprise tasks such as cloning and migrating VMs are not available.

D.
is similar except that objects are at the host and not datacenter level, but typical tasks such as template creation and cloning VMs are available.