Which protocol does an SDN controller use to relay forwarding changes to a southbound API?

OpenFlow is the correct answer because it is a foundational protocol specifically designed for SDN (Software-Defined Networking) controllers to communicate with and control network devices, acting as a southbound API. It allows the controller to directly manipulate the forwarding tables of switches and routers. Java and XML are programming languages and data formats, respectively, not protocols for controller-to-device communication in SDN. While Java might be used to develop controller applications and XML for data representation, neither directly serves as a southbound API for forwarding changes. REST (Representational State Transfer) is an architectural style for networked applications, often used for northbound APIs (controller to applications) or for managing devices, but OpenFlow is the primary protocol for direct forwarding plane manipulation in many SDN implementations.