Final answer:
A controller communicates with applications using Northbound APIs like REST or OSGi to facilitate interactions between network services and application-level logic.
Step-by-step explanation:
A controller communicates with applications in the application plane by using Northbound APIs such as REST or OSGi. The correct answer to the question is option b) Northbound APIs. These APIs provide a way for the applications to request network services from the controller and receive the necessary data to operate effectively. Therefore, they are crucial for enabling communication between the abstracted logic at the application level and the network control layer.