The correct answer is A) the development of agriculture.
The other options of the question were B) stratification of society. C) establishment of trade networks. D) improvement of hunting methods.
The development of agriculture was a direct cause of job specialization in early human civilizations.
This represented a great step in the evolution of humans. Let's remember that ancient humans stopped to be nomads during the Agriculture Revolution or Neolithic Revolution. After following animal herds to hunt them and feed their families, humans developed agriculture techniques that allowed them to settled in on place. That was the case of the oldest human civilization on earth: the Sumerians. They settled in between the Euphrates and Tigris Rivers in what today is Iraq.