The correct answer is B) superior European military technology.
The ability of relatively small European forces to conquer the powerful Aztec and Incan empires can be attributed to multiple factors, including:
A) Lack of immunity to European diseases among Aztec and Incan peoples: European diseases, such as smallpox, to which the indigenous peoples of the Americas had no immunity, decimated their populations, making them more susceptible to European conquest.
C) Ineffective defenses of Aztec and Incan cities: The Aztec and Incan cities were not prepared to defend against the advanced military tactics and weaponry of the European invaders, making it easier for the Europeans to conquer them.
D) Indigenous people's lack of familiarity with horses: Horses were a key advantage for the Europeans, as the indigenous peoples of the Americas had no prior experience or familiarity with horses, which were used effectively by the Europeans in warfare.
E) Successful European missionary activity: European missionaries played a role in facilitating the conquest of the Aztec and Incan empires by converting indigenous peoples to Christianity and gaining their support or cooperation, which weakened the resistance against European invaders.
The only incorrect option listed is B) superior European military technology, as the Europeans did not have superior military technology compared to the Aztecs and Incans. The Europeans did have other advantages, as mentioned above, that contributed to their conquest of these empires.