Final answer:
Northbound API enables an SDN controller to communicate with an application in the application plane.
Step-by-step explanation:
The correct option that enables an SDN controller to communicate with an application in the application plane is Northbound API. Northbound API is an interface that allows the SDN controller to connect and communicate with applications in the upper layer. It provides a set of programming functions and protocols that applications can use to interact with the SDN controller and access network resources.