Final answer:
European actions in the Age of Exploration caused severe population declines in both Africa and the Americas due to the introduction of diseases like smallpox, which indigenous peoples had no immunity against.
Step-by-step explanation:
During the Age of Exploration, European actions similarly affected both Africa and the Americas via the introduction of diseases to indigenous populations. The correct answer to the question is D. Europeans caused populations to decline severely in both places. This significant decline was primarily due to diseases like smallpox and measles, to which the native populations had no immunity. Europeans had developed resistance to many infectious diseases over centuries, but when they came into contact with the peoples of Africa and the Americas, the lack of prior exposure to these diseases resulted in devastating epidemics. The spread of disease significantly contributed to population declines, and its impact ranged from mortalities in the Americas to the introduction of measles significantly reducing the population in places like Fiji.