Final answer:
The primary cause of death among American Indians during European colonization was the introduction of European diseases like smallpox, as Native Americans had no immunity to them.
Step-by-step explanation:
While there were many factors that contributed to the decline of the American Indian population, such as wars, forced removals, and enslavement, the primary cause of death among Native Americans during European exploration and colonization was the introduction of European diseases. These diseases, particularly smallpox, diphtheria, and measles, were devastating due to Native Americans' lack of immunity. But it was not the superior weaponry of Europeans that led to the highest numbers of fatalities; rather, it was infectious diseases that were the most lethal killers.