Final answer:
Windows 8.1 reintroduced the Start button and improved the UI in response to user feedback on the earlier Windows 8 version.
Step-by-step explanation:
The Windows OS that brought back the Start button and made several user interface (UI) refinements was Windows 8.1. This update was in response to user feedback from the initial release of Windows 8, which had a significant departure from traditional desktop interface elements, such as the Start button. After receiving criticism for these changes, Microsoft addressed users' concerns by reintroducing the Start button and improving the UI in the subsequent update, Windows 8.1, released in 2013.