What is a guest operating system (OS)?

The correct answer is: C. The OS that runs inside a virtual environment.

A guest operating system (OS) refers to an operating system that operates within a virtual machine, which is created by a hypervisor. This allows multiple operating systems to run on a single physical machine simultaneously. Each guest OS is isolated from others, which enables users to run different environments, applications, or even different operating systems on the same hardware. If you need more details or further assistance, feel free to ask!