What can cause this issue?

An administrator receives an "HTTP 401 – not authorized" error for the Email Notification Service (ENS). What can cause this issue?
A. A wrong username is set on the ENS server.
B. The Cloud Notification Service (CNS) is unable to authorize ENS.
C. The Apple Push Notification Service (APNS) certificate is not valid.
D. An ENS Service Account is not a member of the local admin group.