Answer:
D. When rivers flooded, they deposited rich silt onto the
surrounding floodplains.
Step-by-step explanation:
Looking at the diagram, the soil-building processes that contributed most to making the shaded area on the map suitable for agriculture is "When rivers flooded, they deposited rich silt onto the surrounding floodplains."
From the diagram, the shaded area is chart resized by Tigris and Euphrates rivers. Both rivers are essential natural features in the area. These rivers served many purposes to the inhabitants, some of which include irrigation systems.
However, these rivers also serve as a silt carrying agent, and they deposit such silt into the surrounding flood Plains.