What types of operating systems are allowable as virtual machine?

A$.$ Windows Server $2012$ R$2,$ for as many instances allowed by licensing

B$.$ Windows Server $2012$ R$2$ and recent down$-$level servers such as Windows Server $2008$ and Windows Server $2008$ R$2$

C$.$Windows operating systems in general

D$.$Several Microsoft and non$-$Microsoft OSs such as CentOS $6\mathrm{.}0-6\mathrm{.}2,$ Red Hat Enterprise Linux $6\mathrm{.}0-6\mathrm{.}2,$ SuSE Linux Enterprise Server $11$ SP$2$