The most accurate option regarding the challenges faced by the Aztec and Inca empires in their encounters with the Spanish is:
D. their armies were too small to defeat the Spanish.
While there were multiple factors that contributed to the fall of these empires, including the technological advantages of the Spanish, the impact of diseases, and the political disunity among indigenous groups, the relative size of their armies compared to the Spanish forces played a significant role in their inability to resist the conquest effectively.