You have a private distribution group with provisioned and unprovisioned iOS devices and need to distribute a new iOS app via Visual Studio App Center. Which option should you choose in App Center to proceed with distribution?

To distribute an iOS app to a private distribution group with both provisioned and unprovisioned devices, you must select 'Register devices and sign my app' in App Center. This option handles the necessary steps for unprovisioned devices by collecting their UDIDs, adding them to your Apple Developer account, and then re-signing the app with an updated provisioning profile that includes these new devices. This ensures all devices in the group can install and run the app. Creating an active subscription in App Center Test is for automated UI testing, not for app distribution. Creating an unsigned build would prevent installation on any iOS device, as all iOS apps require signing. Adding the device owner to the collaborators group grants access to the App Center project but doesn't provision their device or sign the app for installation.