Final answer:
The decline in American Indian populations due to a lack of immunity to European diseases like smallpox most directly resulted in European colonization. Native Americans had never been exposed to such diseases, which made conquest and colonization by Europeans much easier.
Step-by-step explanation:
The most direct result of the change in the American Indian population as described by Las Casas was European colonization. This devastating population decline occurred because Native Americans had no immunity to European diseases such as smallpox, which was responsible for the greatest number of deaths among Amerindian populations in the late fifteenth and early sixteenth centuries. The European diseases, combined with Europeans' conquest and colonization efforts, overwhelmingly decimated Indigenous populations, facilitating European control over the Americas.
The correct answer to the question is D) European colonization. Diseases like smallpox affected Native Americans so badly primarily because of C) Native Americans had no immunity to European diseases. As Europeans arrived in the Americas, they brought with them diseases such as smallpox, against which the Native Americans had no natural resistance, thus experiencing catastrophic death rates that paved the way for European colonization.