The data plane is responsible for forwarding traffic. In traditional devices, the data plane is distributed across the network devices themselves, even when the control and management functions are centralized or influenced by a controller. The control plane decides paths and builds tables; the data plane uses those tables to switch or route packets. The management plane handles configuration, monitoring, and administrative access. A policy plane may describe intent or business policy in some architectures, but it is not the forwarding plane. Cisco CCNA 200-301 v1.1 Automation and Programmability tests software-defined architecture at a conceptual level. The most important separation is that controller-based networking can centralize control while forwarding still occurs on the devices. The plane that is distributed and responsible for traffic forwarding is the data plane. Therefore, D is correct.