Final answer:
The Metro user interface was introduced with the release of Windows 8 in 2012, bringing a new tile-based Start screen to the Windows operating system.
Step-by-step explanation:
The Windows operating system that introduced the Metro user interface (UI) was Windows 8. This was a significant shift in the design language of Windows, focusing on a tile-based Start screen that was optimized for touch as well as mouse and keyboard input.
The Metro UI aimed to make it easier to navigate the operating system by presenting info and applications in a more visually accessible way.
This interface was first seen in Windows Phone 7 and gained prominence with the release of Windows 8 in 2012. Though it was later renamed to "Modern UI," many users and tech enthusiasts continue to refer to it as the Metro UI.