The correct answer to this open question is the following.
Increased agricultural output and the development of craft goods led to the various social classes in ancient Sumer in that with the advent of agriculture in Mesopotamia, people started to earn more material things and increased their wealth. This permanent income made some people be very rich while others stayed as peasants.
With the development of modern agriculture techniques, people started to own portions of land that were farmed by the peasants. Artisans also developed a good way of life followed by the merchants who sold products.
These different labor activities were forming the social classes in ancient Sumer, which later allowed Sumerians to establish powerful city-states such as Eridu, Nippur, Lagash, Kish, or Ur.