What does it indicate when a cluster state is "Active Attention"?

A.
Traffic is being passed, but a problem has been detected: There are no other active members in the cluster

B.
The cluster member is booting: CluserXL is running, but VPN-1/NGX is not yet ready

C.
Cluster members are running different versions: The newer version member is in the ready state, which the older version member is in the active state

D.
Both cluster members are up and ready