What are the three key benefits that application developers should derive from the northbound application-programming interface (API) of software-defined networking (SDN)?

While 'familiar syntax' and 'abstraction of network topology' are correct, 'definition of network protocols' is incorrect because the northbound API abstracts network protocols away from developers rather than defining or exposing them.

'Network syntax' is incorrect because the northbound API is specifically designed to provide familiar (application-oriented) syntax, not network-specific syntax, and 'abstraction of network flow' is not a standard benefit attributed to the northbound API.

'Network syntax' is incorrect for the same reason as B - the API uses familiar programming syntax - and 'abstraction of network commands' is not a recognized key benefit of the northbound API, unlike topology and protocol abstraction.

The northbound API of an SDN controller exposes the network to applications using familiar programming syntax (such as REST or common languages), abstracts the physical network topology so developers don't need to know the underlying infrastructure layout, and abstracts network protocols so applications interact with high-level constructs rather than raw protocol details. This combination allows developers to programmatically control the network without needing deep networking expertise.