The correct answer is:
A combination of superior weaponry and a devastating smallpox outbreak.
Hernán Cortés and his forces used advanced military technology, such as firearms and steel weapons, which gave them a significant advantage over the Aztecs. Additionally, the arrival of smallpox devastated the Aztec population, significantly weakening their ability to resist the Spanish conquest. Cortés also formed alliances with various indigenous groups that were enemies of the Aztecs, which helped him further in his campaign.