What is server virtualization?

A.
Server visualization is the emulation of physical hardware to deliver higher IT utilization and flexibility.

B.
Server virtualization is the simulation of an operating system on physical hardware to deliver higher IT
utilization and flexibility.

C.
Server virtualization is the clustering of multiple servers as a logical entity to deliver higher IT utilization and
flexibility.

D.
Server virtualization is an abstraction layer that decouples the physical hardware from the operating system
to deliver higher IT utilization and flexibility.

Explanation:
http://www.vmware.com/pdf/vi_architecture_wp.pdf (page 3)