Having a ziggurat signified that the city was?
a) in need of protection
b)located on a river
c)large and important
d)small and unimportant

Having a ziggurat signified that city was large and important. 

If any city had a ziggurat, it meant that they were big and important. People in the city believed the ziggurats were the dwelling places of the gods. Each big city had its own patron god, close to human kind.

A: A ziggurat is a place of worship and, unless they asked God to watch over them, it would not protect them

B: has nothing to do with religious worship (In this case)

D: A ziggurat is  not placed in small cities where less worship would take place.

So, your answer is C.