The most accurate answers would be B and C.
B. the Inca and Aztec empires eventually fell under Spanish control because the Spanish had superior weapons. The Spanish had access to advanced weaponry, including guns and cannons, which gave them a significant advantage in battles against the indigenous forces.
C. the Spanish took advantage of civil war. Both the Inca and Aztec empires faced internal strife and civil wars, which weakened them and made it easier for the Spanish to conquer them.
While options A and D contain elements of truth—diseases like smallpox did devastate indigenous populations before and during the Spanish conquest, and there were instances of some initial welcome for the Spanish by certain groups—they do not fully encapsulate the primary factors that led to the Spanish takeover.
So, if you must choose the most relevant, I recommend B and C.