Which feature must be incorporated into the network design to allow seamless Layer 3 roaming between controllers?

Using the same VLANs across controllers would force Layer 2 adjacency and effectively removes the Layer 3 boundary rather than enabling seamless roaming across it.

Mobility groups allow Cisco WLCs to communicate client association state over a mobility tunnel so that when a client roams to an AP joined to a different controller, the anchor controller preserves the original IP assignment. This anchor-foreign relationship is what makes Layer 3 roaming seamless without requiring shared VLANs.

Client load balancing distributes clients across APs to manage AP utilization but has no role in maintaining IP address continuity during a roam.

Link aggregation combines physical uplinks for increased bandwidth and redundancy on a single controller but does not provide any roaming or client mobility function.