The command ' debug mobility handoff enable ' is used to validate the mobility control packets between Wireless LAN Controllers (WLCs). This command enables debugging of the handoff process, which is critical for ensuring seamless client roaming between controllers. When WLCs are separated by firewalls, it is essential to confirm that mobility control packets can traverse these firewalls without being dropped or rejected. The ' debug mobility handoff enable ' command allows network administrators to monitor the handoff process in real time and identify whether mobility PDUs are being exchanged successfully. If mobility control packets are being blocked by the firewall, this debug output will show failures in the handoff sequence. This command directly validates bidirectional mobility traffic flow, making it the correct troubleshooting tool for the scenario described. Options A and D are not valid Cisco WLC commands. Option C enables configuration-level mobility debugging but does not specifically validate handoff packet flow. Reference: WLSD Study Guide — Mobility Troubleshooting, Debug Commands, Inter-Controller Roaming Validation.