The correct answer is A) farming methods.
Many Chinese want to move to Southeast China because of the region's farming methods.
Population in the Southeast of China is denser because many people decide to move to that part of the country. The reason? The land in that part of China is much better for farming purposes. Good, fertile farms in Southeast China are more fertile for agriculture, and China base so much of its economy in agriculture. They also need to feed millions of Chinese people, so its agriculture is a priority.