MS Excel is based on which operating system?

MS Excel is a spreadsheet program developed by Microsoft, and it is part of the Microsoft Office suite of productivity tools. It is available for Windows, Mac, and other platforms.

Excel is based on the Windows operating system, which is a widely used operating system developed by Microsoft. Windows is a graphical user interface (GUI) operating system, which means that it provides a graphical interface for users to interact with the computer and its applications.

Therefore, the correct answer is a) WINDOWS. Excel is based on the Windows operating system.