The correct answer is (a) China.
China has a population of over 1.4 billion people, which is more than the population of the next two most populous countries, India and the United States, combined. China’s population density is 146 people per square kilometer, which is more than twice the population density of the United States and more than three times the population density of India.
Japan has a population of over 126 million people, which is the 11th most populous country in the world. Japan’s population density is 335 people per square kilometer, which is more than three times the population density of the United States but less than half the population density of China.
North Korea has a population of over 25 million people, which is the 55th most populous country in the world. North Korea’s population density is 216 people per square kilometer, which is more than the population density of the United States but less than half the population density of China.
South Korea has a population of over 51 million people, which is the 28th most populous country in the world. South Korea’s population density is 516 people per square kilometer, which is more than the population density of the United States but less than half the population density of China.