Final answer:
Based on the given information, all three areas (Japan, Europe, and the United States) are aging at the same approximate rate in the developed world.The correct option is D.
Step-by-step explanation:
According to the information provided, the text states that many high-income nations are rapidly aging and reaching a point where the elderly will form a larger share of the population. While low-income countries still have a higher proportion of youth and young adults, the elderly populations in these countries are expected to boom by about 2050. Therefore, the most rapidly aging country/area in the developed world based on the given information would be all three areas (Japan, Europe, and the United States).