The correct answer is C) trading between city-states.
The activity that had the biggest impact on economic growth in Sumerian city-states was trading between city-states.
Sumeria was the oldest civilization of planet Earth. They settled in the middle of two rivers in the Middle East region: the Euphrates and the Tigris. It was located in what today is Iraq. There, Sumerians developed important city-states such as Lagash, Nippur, Ur, Eridu, Uruk, and Kish. They developed agriculture techniques and benefited from the fertile soil due to the flooding of the rivers. They established trade between these city-states.