Explanation:

- Option 1, Computer Hardware used: Platform can refer to computer hardware, like processors or physical systems, on which applications run.

- Option 2, Operating System Used: An operating system is often termed a platform as it runs software programs and provides essential services for application programs.

- Option 3, Computer Software Used: Platforms can also include development environments, databases, or applications themselves that provide necessary functions for other applications.

- Option 4, All of The Above: All of these elements (hardware, operating systems, and software) are considered parts of a computing platform as they collectively enable applications to operate.

- Option 5, Both (a) and (b): This option limits the definition to hardware and operating systems, excluding software, which isn't entirely correct.