Cisco Stateful Switchover (SSO) on the Catalyst 9800 platform is the definitive high availability solution when requirements specify zero client reauthentication during failover, active-passive redundancy, and ease of management. With SSO enabled, the active and standby 9800 WLCs maintain a synchronized state database via a dedicated redundancy link — this includes all client association state, authentication credentials, AP join information, and WLAN configurations. When the active controller fails, the standby assumes control instantaneously with no CAPWAP session teardown and no 802.1X reauthentication required. The Redundancy Management Interface (RMI) provides a dedicated in-band keepalive and state synchronization path. N+1 redundancy (Option D) requires APs to rejoin a new controller and clients to reauthenticate, failing the seamless requirement. Manual configuration synchronization (Options A and C) is operationally complex and does not guarantee stateful failover. VRRP provides gateway redundancy at Layer 3 but does not address CAPWAP session continuity. Reference: WLSD Study Guide — Catalyst 9800 High Availability, SSO Architecture, Redundancy Management Interface Configuration.