Of course this is not anywhere near as tough as the real US Citizenship test. This is just to get a feel for how well people know some of the most basic information. They teach all this stuff to kids in school.
I have to admit that I hesitated a little when answering the "most important right" question. Like some of you said, all of them are very important. I guess their thinking is that if you have no say in what your government does, then you are not truly a government of the people.
Just an FYI. Here is a question, along with the answer, from the test linked by Mr Mxy. The actual Citizenship test.
87. What is the most important right granted to U.S. citizens?
The right to vote