Final answer:
The decline of the Aztec Empire after Hernando Cortes' arrival was due to Spanish superior military, strategic alliances with native tribes, and the devastating impact of smallpox on the indigenous population. Option d.
Step-by-step explanation:
The arrival of Hernando Cortes led to the decline of the Aztec Empire mainly because of the superior military tactics and weaponry of the Spanish, as well as the alliances they formed with indigenous peoples who were rivals or enemies of the Aztecs. Furthermore, diseases such as smallpox, introduced by the Spanish, decimated the indigenous population who had no immunity to these foreign pathogens. The combined force of military might, strategic alliances, and disease culminated in the downfall and eventual conquest of the Aztec Empire by 1521.