(b) Operating System
The most important software in any computer is the Operating System (OS), which starts working as soon as we switch on a computer.
Source: Computer Hardware and Software, Chapter 3
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The passage directly states: "The most important software in any computer is the Operating System (OS). This is the software that starts working as soon as we switch on a computer." Examiners expect you to identify option (b) and may also award marks for writing the full form — OS = Operating System.