Having a ziggurat signified that a sumerian city was

in need of protection.
located on a river.
large and important.
small and unimportant.

Having a ziggurat signified that a Sumerian city was (c)large and important 

Ziggurat is a huge ancient building that commonly used to perform huge and important religious ceremony.
The existence of Ziggurat in a city indicates that the City could be regarded as the center of civilization in the past.

The answer to the question is a.