Final answer:
The correct answer is C) Asia, which was the most populous region in the world at the beginning of the 21st century, housing more than one billion people in China and India, the two most populous countries.
Step-by-step explanation:
At the beginning of the 21st century, the region of the world with the highest population was Asia. This is largely because, historically, continental Asia has been dominant in terms of population size. China, in the early 21st century, led with an enormous population, followed closely by India each with more than a billion people. Additionally other Asian countries such as Pakistan also displayed high rates of population growth.
While North America was the most urbanized area and Africa had the highest fertility rates and net population growth rates neither North America nor Africa had a population size to match that of Asia. Therefore the correct answer to the question is C) Asia.