The correct command to restore the running configuration on a Cisco Nexus 5000 Series Switch to an existing snapshot called "stable1" is found in Option C. The command rollback checkpoint stable1 best-effort running-config best-effort ensures that the restore procedure will proceed only if no errors occur, which aligns with the requirement stated in the question. The "best-effort" parameter is crucial as it indicates that the rollback should only proceed if it can be completed without any errors.