Final answer:
The centralized controller uses a northbound API to connect to the Management Plane (C). The correct option is C.
Step-by-step explanation:
A centralized controller in the context of software-defined networking (SDN) typically connects to the network infrastructure using various application programming interfaces (APIs). When such a controller uses a northbound API, it is intended to communicate with the applications and business logic “layer” above it. These applications could be network management systems, analytics, orchestration systems, and more.
The correct answer to which plane a centralized controller connects by using a northbound API is C) Management Plane. The management plane includes functions that control the network equipment for operations, administration, maintenance, and provisioning (OAM&P). This plane is crucial for network management and allows upper-layer applications to define and alter the behavior of the network.
In contrast, the data plane (or forwarding plane) is responsible for the actual forwarding of packets based on the control plane's policy. The control plane, which is directly impacted by the SDN controller, helps direct traffic by setting up the routes and making decisions about where data should go. While both the data and control planes are essential for the function of SDN, they are not directly associated with northbound APIs, which are exclusively for the management plane's use.