Final answer:
A controller communicates with the data plane by using Southbound APIs, with examples including NETCONF, OpenFlow, OpFlex, or OnePK. These APIs facilitate direct management and interaction between the controller and network elements. Option a.
Step-by-step explanation:
A controller communicates with the data plane by using Southbound APIs such as NETCONF, OpenFlow, OpFlex, or OnePK. These APIs allow the controller to manage and interact directly with the network elements such as switches and routers in the Data Plane.
The primary role of southbound interfaces is to provide a means for the controller to send instructions to, and receive information from, the network infrastructure. This is opposed to northbound APIs, which are used for communication between the controller and the applications or business logic layer, enabling network programmability and the creation of network applications.