Final answer:
The strong economic factor for the existence of medieval cities was the economies of scale, particularly in protection and transportation, which come with concentrating economic activity in cities, known as agglomeration economies. The correct option is C.
Step-by-step explanation:
The strong economic factor in the existence of medieval cities is best explained by the concept of economies of scale. Content loaded with this query reveals that these economies were not limited to production within a single firm but were related to the overall growth of population and market in an area. Medieval cities can be seen as agglomeration economies, where grouping economic activity through a dense population and market size was more productive than spreading it out.
When considering the options provided, economies of scale in protection and economies of scale in transportation are strong economic factors for the existence of medieval cities. These agglomeration economies allowed for a more efficient organization of collective defense, reducing the cost of protection on an individual scale. At the same time, concentrating trade and commerce in cities reduced transportation costs and provided better access to goods and services, markets, and input materials.