I wouldn't really think about the 'incans' or 'aztecs' when you talk about empires. I'd suggest to change the question and ask of which american people they conquered the empires.
But really...if the first things you think of when you hear Pizarro and Cortes ISN'T the Incas and Aztecs, you probably wouldn't have gotten it anyways.
They were definitely empires, particularly the Aztecs. Just because you don't know the names of the other people they had subjugated doesn't mean they were all the same people. Actually this was one of the things that made it so easy for Cortes- subjects of the Aztec empire joined in to help in the fight on the Spanish side. The Aztec rulers had been demanding heavy tribute including human slaves for ritual sacrifice from all the neighboring lands that they had conquered.
The Incas conquered an area of 2 millions km², from Colombia to Chile, dominated many other people and imposed their culture. This is exactly what is called an empire.
You should accept Colón (or just Colon) for Christopher Columbus as his real name is Cristóbal Colón. It's also funny that he is considered italian, when at his time Italy didn't even exist. He came from the Republic of Genoa, that was very united to the Crown of Aragon although it wasn't part of it.
Nice quiz. Thanks.
Oh, wait, never mind. I thought it was a thousand years ago there for a second.