The primary purpose of an operating system is
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To make the most efficient use of the computer hardware
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To allow people to use the computer
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To keep systems programmers employed
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To make computers easier to use
The PRIMARY technical purpose of an operating system is efficient hardware resource management (CPU scheduling, memory allocation, I/O handling). While making computers usable (option D) and accessible (option B) are important benefits, they are secondary to resource efficiency.
Classic operating-systems theory (Silberschatz et al.) frames the OS's core job from the system's point of view as managing hardware resources — CPU, memory, I/O, storage — as efficiently as possible, arbitrating among competing processes. 'Allow people to use the computer' and 'make computers easier to use' describe the user-convenience view, which is a secondary goal, not the primary one measured by exam conventions. 'Keep systems programmers employed' is a joke distractor with no technical basis. So 'efficient use of hardware' is the standard textbook answer for the OS's primary (resource-management) purpose.