Final answer:
The factor contributing the most to China's high number of Internet users is its large population size. Despite government control over the Internet content, the vast population and economic growth initiatives like SEZs have led to a massive number of Chinese citizens being connected online.
Step-by-step explanation:
Of all the countries, China has the most Internet users with nearly 400 million people. The primary factor that contributes to China having the highest number of Internet users is C) Population size. With over 1.33 billion people as of 2010, China has more people than any other country in the world, and most of them live in China Proper, which comprises the eastern regions with the most fertile lands and moderate climates. China's large population alone creates a vast user base for the Internet. Moreover, the considerable rural-to-urban shift and economic growth, fueled by policies such as the creation of special economic zones (SEZs), have helped to increase Internet use among Chinese citizens. Despite the government's control over the Internet, described as the suppression of 'harmful information,' the sheer volume of people means that a significant number are connected online. China's large population is a significant factor in its having the most Internet users. With nearly 1.4 billion people, even a moderate percentage of the population using the Internet results in a substantial absolute number of users. While government policies (A), economic factors (B), and technological infrastructure (D) also play roles, the sheer size of China's population has a major impact on its Internet user numbers.