The correct answer is A. Monsoons.
Even though the monsoons appear in multiple locations apart from South Asia, like Southeast Asia, eastern and southern part of China, and even along the coast of East Africa, still the most heavily influenced region by them is South Asia. Monsoons are crucial for the environment in South Asia, as well as for the humans, but it also can be a destructive force that takes lots of lives and destroys lots of homes too. The monsoons in here appear in the summer, they are the biggest producer of rainfall on Earth, and all the world records in biggest precipitation are in India, mostly in the part where the Himalayas and the plains meet. In places it is recorded rainfall of over 12,000 mm. These rains allow the existence of big rivers, which in turn flood because of the huge amount of water and deposit huge amounts of fertile materials in the plains, that makes the soil incredibly fertile and the people can have excellent results in agriculture.