Which of the following was a mobile device developed by Microsoft?

The choice of Windows Phone 7 is correct because it is directly associated with Microsoft's development of a mobile operating system specifically designed to compete in the smartphone market. Released in 2010, Windows Phone 7 was part of a significant shift in Microsoft's mobile strategy and marked a departure from Windows Mobile, with a modern user interface and integrated services.

Windows Phone 7 introduced features like Live Tiles and a focus on a seamless experience across devices, aiming to capture a share of the growing smartphone market against competitors like Apple's iOS and Google's Android. This makes it a defining product in Microsoft's mobile history.

Other choices, while related to Microsoft's mobile endeavors, do not represent the same pivotal launch as Windows Phone 7 did. For instance, Windows CE was more of a real-time operating system used in embedded systems and earlier mobile devices but wasn't a dedicated smartphone platform like Windows Phone 7. Windows Phone 6 also fits into the older Windows Mobile category, which was phased out in favor of the newer Windows Phone series. Windows Vista Mobile is not a recognized product; the naming conventions did not proceed to create a version reflecting Vista in the mobile realm.