A northbound API lets applications communicate with the controller. In controller-based networking, the controller sits between applications and the network devices. Applications use northbound APIs to request services, retrieve information, or express intent. The controller then translates those requests into device-level actions through southbound communication. This is the direction model CCNA candidates must keep straight:
northbound faces applications and orchestration systems; southbound faces routers, switches, access points, and other infrastructure. A northbound API does not directly communicate with physical hardware, and it is not primarily a mechanism for raw device error reporting. It can expose statistics, but that is not its defining purpose. Cisco CCNA 200-301 v1.1 covers northbound and southbound APIs under Automation and Programmability because modern network operations increasingly use controllers and REST-style interfaces.
The answer that precisely describes the northbound role is D: it facilitates communication between the controller and the applications.