Final answer:
OpFlex is the southbound API that uses a declarative SDN model, focusing on desired outcomes rather than prescriptive instructions.
Step-by-step explanation:
The southbound API that matches the description of using a declarative SDN (Software Defined Networking) model is OpFlex. Unlike imperative models where detailed instructions are provided to networking components, a declarative model like OpFlex specifies the desired outcome, and the network devices determine how to achieve that outcome. This approach is different from OpenFlow, which is more prescriptive, and NETCONF, which is a protocol used for network configuration. OpFlex communicates policies that are enforced by network devices, allowing for a more dynamic and flexible SDN environment.