The correct answer is - D. decreasing populations.
The Middle East is not a region that has a problem with decreasing populations, nor it is a factor that in any way affects the water shortages in the region, if anything, there is an ever-growing population in the region.
The shortages of water in the Middle East are mainly because of climate factors. It is a very dry region, where in general the landscape is dominated by deserts and semi-arid steppes. The precipitation in most of the region is very low, so that contributes to small amount of rivers, thus very reduced amount of fresh water. Some of the countries are trying to solve this problem with desalinization of the sea water.
