Refer to the diagram. Which solution must be used to send traffic from the foreign wireless LAN controller to the anchor wireless LAN controller?

MPLS VPN or VRF is a routing/segmentation technology and is not the encapsulation method used between WLC peers in the mobility anchor architecture.

Sending packets without encapsulation would not preserve the client's Layer 2 identity across a routed network, breaking the mobility anchor functionality.

Cisco WLC mobility anchoring specifically uses EoIP (Ethernet over IP) tunnels to carry client traffic between foreign and anchor controllers. This encapsulation preserves the original Ethernet frame, allowing the anchor controller to apply policy and provide the client with its home VLAN/subnet even when roaming to a foreign site. EoIP is the defined inter-controller tunneling mechanism in Cisco's mobility architecture.

IPinIP or IPsec are valid inter-site tunneling protocols in general networking, but Cisco WLC mobility specifically requires EoIP encapsulation, not these protocols, for controller-to-controller client traffic.