In a software-defined network (SDN), the control plane and data plane are decoupled, allowing for more flexible network management and control. This is in contrast to traditional networks where the control plane and data plane are tightly coupled (A), which is not a foundation of SDN. SDNs are indeed built upon a robust software stack (B), which allows for centralized management and dynamic resource allocation. Traditional networks often require device by device-level configurations, but this is not a foundation of SDN; instead, SDNs aim to simplify network management through automation. Automated through expressed intent to a software controller (D) is another key feature of SDN, as it enables the network to be programmatically configured and managed through higher-level policies rather than manual device-specific configurations.
Lastly, requiring significant physical hardware resources (E) does not distinguish an SDN from a traditional network as both can be resource-intensive depending on their scale and function. References: = Automating Cisco Enterprise Solutions Official Cert Guide