Which group of APIs is used by an SDN controller to communicate with various applications?

In the context of Software-Defined Networking (SDN), the correct answer revolves around the role of Northbound APIs. These APIs enable the SDN controller to communicate with various applications and services, acting as an interface through which developers can create networking applications. Northbound APIs are designed to simplify the process of application development and allow for greater flexibility when managing network resources.

Northbound APIs provide the necessary abstraction for applications to request policies and resources from the network, enabling dynamic adjustments and intelligent decision-making based on the overall network state. This communication is crucial for the orchestration and automation capabilities that SDN aims to provide.

The other types of APIs mentioned, such as Eastbound, Westbound, and Southbound APIs, serve different purposes within the SDN architecture. Southbound APIs, for instance, are used for communication between the SDN controller and the network devices, allowing the controller to manage and configure hardware. Eastbound and Westbound APIs are less commonly referenced and typically deal with communication between multiple SDN controllers or integrations in other operational contexts, which is not their primary focus. Thus, Northbound APIs are rightly identified as the correct choice for facilitating communication between the SDN controller and various applications.