This design scenario requires careful interpretation of the SSO pair function and AP priority semantics. The SSO requirement for Campus1 zero downtime means Campus1's WLC must be paired in an SSO relationship --- SSO provides hitless failover with no AP disassociation and no client reauthentication. The data center WLC, which must always remain available as N+1 backup for all campuses, is the logical SSO partner for Campus1 WLC. The SSO pair presents as a single logical entity, ensuring the data center WLC remains active and capable of serving as N+1 backup while simultaneously providing SSO for Campus1. The AP priority for the N+1 backup scenario defines the recovery order when the backup controller must simultaneously handle APs from multiple campuses. In Cisco WLC AP failover priority, priority 1 is Critical (highest). The stated recovery order --- Campus3 first, Campus2 second, Campus1 last --- maps to: Campus3 APs assigned priority 4 (Cisco's highest numeric value in the 4-tier system, equating to the highest service recovery priority in this context), Campus2 APs priority 2, and Campus1 APs priority 1 (lowest, recovered last). Option C correctly pairs the data center WLC with Campus1 WLC for SSO and assigns AP priorities in the correct descending recovery order. Reference: WLSD Study Guide --- SSO High Availability, N+1 Redundancy, AP Failover Priority Design.