Final answer:
The term that best describes the software or hardware used to manage virtual machines is 'Hypervisor'. It allows for multiple OS to coexist on a single physical host by managing resources and segregating each virtual environment.
Step-by-step explanation:
The term that BEST describes the software or hardware used to manage virtual machines is A. Hypervisor. A hypervisor, also known as a virtual machine monitor (VMM), is a piece of software, firmware, or hardware that creates and runs virtual machines (VMs). It allows multiple operating systems to share a single hardware host, with each operating system and its applications running within a separate virtual machine. Hypervisors provide a layer of abstraction between the hardware and the virtual environment and are responsible for managing the system's processor, memory, and other resources to allocate them properly to each VM.