China has the largest population in the world, with over 1.4 billion people. Japan has a much smaller population (around 125 million), Cuba has approximately 11 million people, and London is a city (not a country) with about 9 million people. The question asks for the country with the most people, making China the clear correct answer. Note that as of 2024, India's population has surpassed China's, but among the given options, China remains the correct choice.