Answer:
C
Step-by-step explanation:
Using process of elimination and that 5 on the AP World exam, you can conclude that:
A is not a suitable answer because that's more of a climate issue than a geographic on.
B is not suitable either because the location has incredibly rich and fertile soil, in fact, their crops (barley and wheat) grew well in this region.
D is not suitable because Sumerians were susceptible to outside attacks, something mountains would protect them from.
Now, C is the correct answer by default but also because Tigris and Euphrates were irregularly flooding which would hurt their civilization and the flooding also reflected what kind of society they had which was unstable compared to the Egyptians who had a strong, central rule that matched the Nile river that flooded very regularly to the point they could calculate when it arrived.