What defines a guest machine in virtualization?

A guest machine in virtualization refers to an independent instance of an operating system that is created and managed by a virtual machine monitor (also known as a hypervisor). In this context, the guest machine operates as though it is a physical computer, but it actually runs on virtualized hardware provided by the host machine.

The key aspect of a guest machine is its ability to function independently from the host operating system, allowing it to run its own applications and services. By using virtualization technology, multiple guest machines can coexist on a single physical machine, enabling better resource utilization and flexibility in computing environments.

The other choices describe different concepts that do not directly relate to the definition of a guest machine. For instance, the primary operating system installed on a host refers to the host OS itself, not a guest instance. A shared application across multiple users can refer to software that is accessed by users over a network but does not describe a virtual machine. A resource management tool for servers pertains to methods and applications for managing server resources, which again is not specific to the virtualization of operating systems.