For Microsoft, which operating system did the company first release in 1985?

Microsoft first released Windows 1.0 in 1985.

Windows 1.0 launched on November 20, 1985, as a graphical operating environment for MS-DOS rather than a completely independent operating system. It introduced features such as tiled windows, menus, icons, dialog boxes, and support for a mouse.

The product was originally developed under the codename Interface Manager. Microsoft changed the name to Windows before release, reflecting its central use of on-screen windows. Early versions included applications such as Calculator, Paint, Notepad, and the first Windows version of Control Panel.

A common mix-up is confusing Windows 1.0 with MS-DOS. MS-DOS was Microsoft's command-line operating system, while Windows 1.0 provided a graphical layer that ran on top of it. Later releases, especially Windows 3.0 and Windows 95, became much more widely used, but Windows 1.0 established the product line that became central to Microsoft's software business.