The primary purpose of an operating system is

Operating-system theory presents two goals side by side: the user view — convenience, making the computer easy to use — and the system view — efficient resource management, allocating the CPU, memory, and I/O devices fairly among competing processes.

When a single-answer question asks for the OS's ‘primary purpose’, it is testing which of these two views is treated as foundational. For this exact ISRO CS 2009 item, independently published answer keys converge on the system view: efficient use of the computer hardware. The reasoning is that resource management is the enabling layer — it is what lets any program run at all — while ease of use is a benefit built on top of that management rather than a replacement for it.

• To make computers easier to use — a genuine OS benefit, but it names a user-facing outcome that rests on resource management rather than the foundational, system-level function itself.

• To allow people to use the computer — too generic to name any actual OS mechanism (scheduling, memory allocation, device management).

• To keep systems programmers employed — not a real system objective; it can be eliminated outright.

Because independently published answer keys for this specific PYQ converge on efficient hardware use, and the other two options fail to describe any real OS objective, efficient use of the computer hardware is the defensible answer here — with ease of use standing as the closely related, but secondary, benefit.