Which technology is used for communication between an SDN controller and network applications?

REST API is the correct answer. Network applications communicate with the SDN controller using northbound APIs, which are typically RESTful APIs. These APIs allow applications to request network services, retrieve network information, and program network behavior. Southbound API is incorrect because it describes the communication between the SDN controller and the network devices (e.g., switches, routers), not applications. OpenFlow is a common southbound API protocol. NETCONF is incorrect. While NETCONF is a protocol used for configuring network devices, it's primarily used for device management and configuration, not as the primary interface for applications to interact with an SDN controller for network programmability. OpenFlow is incorrect because it is a southbound API, used for communication between the controller and the data plane devices, not between applications and the controller.