Answer:
Firstly,
we shall see What is OS?
OS(Operating System) is a collection of program that manage computer system's internal work like its memory, processors, devices and file system. Operating system are designed to make the best use of the computer's various resources and ensure that the maximum amount of work is processed as efficiently as possible. An OS cannot increase the speed of a computer it just can maximize use of resources.
Most of mainframe computers are design by IBM.So, I will prefer z/OS which is IBM's foremost mainframe operating system. it is useful for mainframe students, however, to have working knowledge of other mainframe operating system.One reason is that a given mainframe computer might run multiple operating systems.
z/OS, a widely used mainframe operating system is designed to offer a stables, secure and continuously available environment for applications running on the mainframe.