The correct answer is C . In FortiNAC-F Manager clustering, the active cluster management role is the leader role, and other cluster members operate as worker nodes. Fortinet's FortiNAC-F 7.6 Manager Cluster Guide states that when a worker is promoted to leader during automatic transition, the original leader becomes a worker node when it comes back online. It also explains that a cluster has one leader and can have multiple workers, with leader/worker roles used for cluster management and heartbeat exchange.
So, if the cluster manager becomes non-responsive and another node takes over leadership, the recovered manager does not automatically reclaim the leader role. It rejoins as a worker node. Option A is wrong because recovery does not automatically remove it into a standalone group. Option B is wrong because automatic return to the previous manager/leader state would create instability and possible split-brain behavior. Option D is wrong because "standby" is not the role described for FortiNAC-F Manager clustering in this context; the documented cluster roles are leader and worker.