Which technology allows a standby supervisor to assume control when the primary supervisor fails?

Stateful Switchover (SSO) is the correct answer because it allows a standby supervisor engine to take over control from a failed active supervisor without interruption to forwarding or loss of state information for established connections. This ensures high availability and minimal downtime. FHRP (First Hop Redundancy Protocol) is incorrect because it provides redundancy for the default gateway, not for supervisor engine failures within a single device. Graceful restart is incorrect as it's a routing protocol feature that allows a router to restart without disrupting forwarding, but it doesn't address supervisor engine failover. NSF (Nonstop Forwarding) is incorrect because it works in conjunction with graceful restart to maintain forwarding during routing protocol restarts, but like graceful restart, it doesn't directly handle supervisor engine failover.