The correct answer is D, domestication of animals. All other alternatives refer to events that only happened later.
In order to affirm an Agricultural Revolution ocurred, historians have to choose the most important changes and events that determine that from that moment onwards life was different.
The most important thing that supports the idea of an Agricultural Revolution is that humans started to domesticate plants and animals, which made possible for them to settle and stop being nomads.
This lead to the birth of cities and to radically change how those people lived.