Which SDN architecture component is used by the application layer to communicate with the control plane layer to provide instructions about the resources required by applications?

Southbound APIs are used in the opposite direction - between the control plane (SDN controller) and the data plane (physical and virtual switches/routers) to program forwarding rules.

Northbound APIs sit above the SDN controller and expose the network's capabilities upward to business applications and orchestration systems. They allow application-layer programs to communicate traffic policies, bandwidth requirements, and service chaining instructions to the control plane without needing direct knowledge of underlying hardware.

The orchestration layer sits above the application layer and coordinates multi-domain or multi-service workflows; it does not describe the interface between the application layer and control plane.

The SDN controller is the control plane component itself that receives instructions - it is not the API interface used to deliver those instructions.