Final answer:
In 2008, half of the world's population lived in cities, which is option d) 50%. This reflects the global urbanization trend that has grown since 1900 and poses sustainability challenges.
Step-by-step explanation:
In 2008, the percentage of the world's population living in cities was 50%. Throughout history, there has been a significant urbanization trend correlating with economic development. In 1900, only 20% of the global population lived in urban areas, which doubled to 40% by 1989. By the 21st century, the urbanization process had intensified, with more than half of the global population residing in urban settings. This shift has been more pronounced in developed regions such as North America and Europe, while areas such as Africa had and still have a lower percentage of urban dwellers. The continued trend of urban growth presents challenges for sustainable development, especially in less developed countries.