In the middle east, a is a dry riverbed that often fills briefly with water after strong rains.
a. nafud
b. wadi
c. shamal
d. dam

The correct answer is letter B.  

Wadis are underground areas that have the ability to contain and provide water sources. It is an Arabic word which means “valley” or “canyon.” It’s usually found in arid parts of the world like the Middle East. These things generally fill with water after a season of heavy rainfall which serves as a major source of water for the inhabitants in its area.