The correct answer is: B. Microsoft Hyper-V
Microsoft Hyper-V is a hypervisor that enables you to create and run multiple operating systems on a single physical computer. It is a full-featured hypervisor that supports both x86 and x64 guest operating systems. Hyper-V is included in Windows Server 2008 R2 and later versions.
VirtualLogix VLX is a hypervisor that enables you to create and run multiple operating systems on a single physical computer. It is a full-featured hypervisor that supports both x86 and x64 guest operating systems. VLX is not included in Windows Server.
Xen is a hypervisor that enables you to create and run multiple operating systems on a single physical computer. It is a full-featured hypervisor that supports both x86 and x64 guest operating systems. Xen is not included in Windows Server.
Therefore, the only option that is a host operating system for Windows Server is Microsoft Hyper-V.