What was the key factor in the development of cities? the origin of large scale warfare that required greater defense of the population. the development of more efficient agriculture. the rise of government dedicated to the needs of the people. the establishment of organized religion?

the best option is B
the development of more efficient agriculture
 
The emergence of efficient forms of agriculture especially in Mesopotamia, led to emergence of early city states as food was produced in surplus to feed the town population. it also led to division of labor. this form of agriculture also contributed to sedentary lifestyle and improved nutrition thus explosive population that contributed to city establishments.