Which of the following allows multiple operating systems (guests) to run concurrently on a host computer?

An emulator mimics the hardware and software of one system on another, allowing software written for one platform to run on a different one, but it's not primarily for running multiple distinct operating systems concurrently as guests.

A hypervisor, also known as a Virtual Machine Monitor (VMM), is a software, firmware, or hardware that creates and runs virtual machines (VMs). It allows multiple guest operating systems to share the host computer's hardware resources while running isolated from each other.

Grid computing is a distributed computing paradigm where a 'grid' of computers work together to solve a large computational problem, not for hosting multiple operating systems on a single machine.

CP/CMS (Control Program/Cambridge Monitor System) was an early IBM operating system that included virtualization capabilities, but 'hypervisor' is the general and more current term for the technology described.