All you have to do to get this answer is to look in the Constitution in the back of your history book. All the amendments are listed and they are not really all that long. If you look there, you will see that the 26th Amendment was the one that gave people 18 and over the right to vote.
This amendment was ratified in 1971. It is generally believed that the amendment was able to pass because of the Vietnam War. Most people would say that public opinion came to favor allowing 18 year olds to vote because they were being drafted and sent to fight in Vietnam. If they were old enough to fight for their country, the logic went, they were certainly old enough to be allowed to vote.
For this reason, 18 year olds were given the right to vote. The correct answer is B.
b) 26th Amendment