If the user is unaware that the mobile device's OS version is not current despite auto-update being turned on, the most likely cause of the issue is that the device is restricted from updating due to a corporate security policy, option D. Corporate-managed mobile devices are often subject to specific security policies and configurations set by the organization to protect sensitive data and resources. It is possible that the corporate security policy restricts OS updates to ensure compatibility with the organization's security protocols and prevent unauthorized access.
Option A, not enough free space, could cause problems with downloading and installing updates, but it would not cause the user to be unaware of the outdated OS version.