Correct answer is spreading of smallpox and some other diseases that Natives were not immune to, which eventually practically led to their extinction.
Explanation:
When the conquistadores started conquering lands of Aztecs and Incas they did it by power of weapons, which Natives didn't have. But, something that was even a greater problem for them was when diseases started spreading among them. Of course, Europeans never intended to spread this diseases, but obviously inadvertently spread them among Natives.
