Explanation:

This question requires identifying which of the given options is not an operating system. Let's analyze each option: (a) macOS is an operating system developed by Apple Inc. for its Macintosh line of computers. (c) MS Windows (Microsoft Windows) is a family of graphical operating systems developed and marketed by Microsoft. (d) Linux is a family of open-source Unix-like operating systems based on the Linux kernel. All three (macOS, MS Windows, Linux) are indeed operating systems. However, (b) Microsoft Office is a suite of office productivity applications developed by Microsoft, including programs like Word, Excel, PowerPoint, and Outlook. It is application software, not an operating system. Therefore, Microsoft Office is the correct answer as it is not an operating system.