Religion is the answer; education, money, and occupation determine what class you are in.
You allready got the answers to this
Your A and B are the same, so I assume you messed up there.
But the 26th amendment granted the right to vote to American citizens aged eighteen or older, or 18+.
The "d. United Nations" would not be a good example of an NGO, since NGOs (non-governmental organizations) usually act independently and on a much smaller scale.
According to the Ninth Amendment, it is the "people" of the states who retain rights not specifically listed in the Constitution, since the Founding Fathers wanted the new government to protect individual liberty as much as possible.