Final answer:
The monsoon is a seasonal reversal of winds that brings heavy rains to South Asia. It plays a crucial role in supporting the region's large population by providing water for agriculture.
Step-by-step explanation:
The correct answer is A. Monsoon. A monsoon is a seasonal reversal of winds that brings heavy rains. In South Asia, the summer monsoon brings moist air from the Bay of Bengal and the Indian Ocean, resulting in rainfall between June and September. In the winter, the cold, dry air blows to the south, creating the winter monsoon. The monsoon is a major source of support for South Asia's large population as it provides water for agricultural production.