The correct answer is - d.) The treatment of conquered people.
The Aztecs were very cruel and aggressive when it come to the treatment of the conquered people. Their basics methods of ruling were force and threatening with force. Because of this, the conquered people despised the Aztecs, and just waited for a suitable situation to get rid of them.
The arrival of the Spanish and their intentions for the Aztecs were the perfect opportunity. The Spanish would probably not have been able to win against the Aztecs one on one, but they got a lot of recruitment from the people that were conquered by the Aztecs. They wanted to rebel and gave their best, they were leading the Spanish as they new the terrain perfectly, had knowledge of the Aztecs military tactics, and they were also the intelligence on the battlefield.