What is the function of a controller in controller-based (SDN) networking?

In Software-Defined Networking (SDN), the controller is the brain of the network. It acts as the centralized management and control point, translating high-level policies into network-specific configurations and forwarding rules for the underlying network devices (data plane). This separation of the control plane from the data plane is a core tenet of SDN. "A pair of core routers that make all campus routing decisions" is incorrect because traditional routers combine control and data planes, which is the opposite of SDN's philosophy. "Centralizes the network data plane" is incorrect as the data plane remains distributed across network devices; only the control plane is centralized. "A router line card that maintains all campus routing decisions" is incorrect because a line card is a component of a single router, not a centralized network-wide control point.