Final answer:
In an SDN architecture, southbound protocols use an Application Programming Interface (API) to allow the SDN controller to manage the network's physical devices.
Step-by-step explanation:
Southbound protocols in a Software Defined Networking (SDN) architecture connect the SDN controller to the network's physical devices. The correct answer to the question is b. API, which stands for Application Programming Interface.
APIs facilitate the communication between software applications, which in the context of SDN, allows the controller to manage the network devices. The API provides a set of routines and protocols that the SDN controller uses to communicate with the switches and routers, instructing them on how to handle network traffic.