In northern europe, many reclaimed polders, such as the one in the photo above, are created by systems of that have been built to hold back water.
a. fjords
b. foehns
c. siroccos
d. dikes

The correct answer is letter D. Dikes or Dykes can be natural or artificial and their purpose is to regulate water levels. They are usually made of stone walls and can also serve as drainage trenches for water found in lakes and dams. 

Iwould say the answer would be b

South central region