Don't get caught having partines
To be honest, I dont know. I'm actually not an American. :)
But If i had to pick one major event...it would be making George Washington the president.
To explane.
George Washington's presidentsy in a way made America, in some aspects, what it is today.
Now imagine if he was not made a president but chose to be a dictator. I know its funny and my nest statement is going to be even funnier.
Assassins Creed 3: The rule of King George did a decadent job in imagining and explaning this. I recommend reading its plot, I think you would find it interesting. Its full of what ifs and alternative history events.
Sorry for not giving my version but its a bit hard for me to imagine something like that.
B, C (lol B, C like Before Christ xD)
D. Castiglione
Cesare Borgia was the Son of the Pope and Cosimo de Medici's family ruled Florence, so neither have anything to do with the royal family of Mantua. Machiavelli wrote the Prince, so that is not the correct answer either. Castiglione was part of the Royal family of Mantua and did write the Book of the Courtier. So D is the correct answer.