Centralized Plane

D.

An SDN (Software-Defined Networking) controller centralizes the control-plane in a network infrastructure, meaning that the answer is D. control-plane.

The control-plane is responsible for establishing and maintaining the network's communication paths, routing protocols, and forwarding tables. This plane communicates with the management-plane (A), which is responsible for monitoring and managing the network devices, and the data-plane (B), which is responsible for forwarding the network traffic.

The services-plane (C) is a plane that is specific to certain devices, and it handles specialized services, such as deep packet inspection or application visibility and control.

In summary, the SDN controller centralizes the control-plane, which is responsible for managing the network's communication paths and forwarding tables. It communicates with the management-plane, which monitors and manages network devices, and the data-plane, which is responsible for forwarding network traffic.