The answer is Roll back to the original image because Cisco Nexus and ACI forwarding depends on exact agreement between the devices participating in the feature. The scenario references FEX; those technologies fail in predictable ways when required parameters do not match or when the required control-plane advertisement is absent. In DCIT troubleshooting, the engineer should identify the failed dependency, correct that dependency, and then verify the operational table that proves the fix. For example, vPC and EVPN checks focus on consistency, VNI/VRF bindings, peer-gateway behavior, and host-route advertisement; routing checks focus on neighbor state, MTU, timers, authentication, next-hop, and route preference. The selected answer addresses the dependency that blocks forwarding. The distractors are plausible because they touch related features, but they do not resolve the specific failure condition. Post-change validation should include protocol state, learned endpoint or route entries, and a traffic test across the affected path.