Which Cisco WLC feature lets a client perform a Layer 3 roam between controllers without changing the client IP address?

A mobility tunnel (specifically, a Layer 3 mobility tunnel) is the correct answer because it allows a client to roam between different Wireless LAN Controllers (WLCs) that are in different Layer 3 subnets while maintaining its original IP address. This is achieved by tunneling the client's traffic between the anchor WLC (where the client initially connected) and the foreign WLC (where the client roams). Mobile IP is a general protocol for host mobility but is not the specific Cisco WLC feature for inter-controller Layer 3 roaming. LWAPP (Lightweight Access Point Protocol) is an older protocol for AP-WLC communication, not for client roaming between WLCs. GRE (Generic Routing Encapsulation) tunnels are a general-purpose tunneling mechanism, but "mobility tunnel" is the specific Cisco WLC implementation that leverages tunneling for this purpose.