The correct answer is the "votes of all thirteen states."
In order to add an amendment to the Articles of Confederation, all thirteen states had to agree on it. This proved to be extremely difficult and was one of the many flaws of the Articles of Confederation. Since the federal government had no significant power, they sometimes weren't even able to get all of the representatives to be there to vote. This is why this constitution was replaced with the US Constitution after less than a decade.
