Final answer:
Spanish explorers brought diseases that devastated the Aztec population.
Step-by-step explanation:
The factor that most contributed to Hernán Cortés's victory over the Aztec Empire was B. Spanish explorers brought diseases that devastated the Aztec population.
The inadvertent introduction of smallpox into Tenochtitlan resulted in the deaths of thousands in the city. The Aztec Empire was already weakened due to the spread of infectious diseases brought by the Spanish, and this further weakened their military and population, making it easier for Cortés to defeat them.
Overall, the combination of military tactics, weaponry, and diseases brought by the Spanish played a significant role in Cortés's victory over the Aztec Empire.