An operating system is the software layer that manages a computer's hardware and resources so that application programs can run correctly. It performs several distinct management jobs at the same time, not just one.
Memory management controls how the computer's RAM is allocated to running programs; file management organises how data is stored, named, and retrieved on storage devices; and application management controls how programs are started, run, and closed. Since an operating system genuinely performs all of these tasks together, and none of them alone fully describes what an operating system does, the combined answer is the accurate one.
Choosing only one of the three individual tasks would understate the operating system's role, because a real operating system such as Windows, macOS, or Linux is responsible for coordinating memory, files, and applications simultaneously, not for just one of these functions in isolation.
When a question lists several genuinely correct, non-overlapping functions of the same system and then offers a combined answer that covers them all, check whether the system in question is broad enough to perform all of them, as an operating system is.
