Final answer:
The primary cause of subtropical deserts is subsiding air masses that form high-pressure zones, leading to very dry conditions with little precipitation, commonly found around 30° north and south latitude. The correct answer is option b.
Step-by-step explanation:
The primary cause of subtropical deserts around the world is b. subsiding air masses. These air masses descend in the subtropics around 30° north and south latitude, forming high-pressure zones that lead to clear, dry conditions and very little precipitation. The subsiding air warms as it descends, absorbing any remaining moisture from the environment, which significantly reduces rainfall. This dry condition is exacerbated by the presence of mountain ranges that can create rain shadows, further decreasing the likelihood of precipitation on the leeward side where deserts like the Mohave and Sonoran are located. The Sahara Desert and the Namib Desert are classic examples of such arid regions formed due to the high-pressure, dry air descending at these latitudes. In these deserts, evaporation typically exceeds precipitation, leading to very dry conditions with low annual rainfall that can sometimes be as little as 2 cm (0.8 in).