When is a 64-bit Windows Operating System required for the installation of vCenter Server?

When is a 64-bit Windows Operating System required for the installation of vCenter Server?

A.
When the managed vSphere environment exceeds 200 ESX hosts

B.
When the managed vSphere environment exceeds 1000 virtual machines

C.
When deploying vSphere 4.1 or later

D.
vCenter Server is not supported on 64-bit Windows operating systems