What do you call software that emulates a particular computer system?

What do you call software that emulates a particular computer system?

• Guest operating system
• Virtual machine
• Hypervisor
• Processor 

EXPLANATION

 In computing, an emulator is hardware or software that enables one computer system (called the host) to behave like another computer system (called the guest). An emulator typically enables the host system to run software or use peripheral devices designed for the guest system. Emulation refers to the ability of a computer program

The software layer that emulates the necessary hardware for an operating system to run in. It is the hardware virtualization that lets multiple operating systems run simultaneously on a single computer, such as a network server.