Explanation:

The ability of an operating system (OS) to execute multiple tasks or applications concurrently over a period of time is called multitasking. Even on a single-core processor, the OS rapidly switches between tasks, giving the illusion that they are running simultaneously. Option (a) 'multitasking' precisely describes this capability. Option (b) 'object-oriented programming' is a programming paradigm, not an OS feature. Option (c) 'multi-user computing' refers to the ability of an OS to allow multiple users to access and use a single computer system simultaneously, which is distinct from a single user running multiple applications. Option (d) 'time-sharing' is a technique or mechanism used by operating systems to achieve multitasking by allocating CPU time slices to different processes, but 'multitasking' is the overarching term for the ability to run multiple applications concurrently.