Final answer:
Most deserts are found around 30° latitude north and south due to the high-pressure zones created by the descending air of the Hadley cells, which drop hot, dry air, making conditions arid.
Step-by-step explanation:
The deserts are present at around 30° latitude north and south largely because of the global wind and pressure patterns associated with the Hadley cells. In these regions, the descending limb of the Hadley cell creates a zone of high pressure where the air is compressed and heated, reducing its ability to hold moisture and thus creating an arid environment. This process, combined with the dynamics of rain shadows created by mountain ranges and the overall low precipitation levels due to warm, dry air descending, leads to the formation of subtropical deserts.
The correct answer to the question is: 'a. the Hadley cells in the summer drop hot dry air at those latitudes' because the Hadley cells consistently influence global climate, bringing dry and hot air down to these latitudes all year round, not just in one season. These areas experience high evaporation rates, leading to very dry conditions with little precipitation.