Apple Certified Support Professional Practice Test

Using iCloud is a valid method for backing up an iOS device before decommissioning it. iCloud allows users to securely store data like photos, videos, app data, device settings, and more in the cloud. Before decommissioning, a complete backup ensures that the user can restore their data to a new device or access it later if needed. This method is particularly convenient as it can be performed wirelessly and does not require any physical connections to a computer, making it accessible and easy to use.

In contrast, uninstalling apps does not serve as a backup method; it simply removes applications from the device without preserving any associated data. Running a diagnostic is more about checking the health and performance of the device rather than creating a backup. Finally, erasing all contents would remove all data from the iOS device, making it a necessary step in decommissioning but not a backup solution. Thus, using iCloud stands out as the appropriate choice for ensuring that relevant data is preserved prior to decommissioning the device.