Answer: The answer is "The first time the Constitution defined what a citizen is". This is very important because not only is the first time the Constitution set down any hard and fast rules about who were "official" citizens of the United States, but also ties in the rights that citizens have which can not be infringed on. Being passed shortly after the end of the Civil War, it was rather hotly contested, especially since, among other things, former slaves would now officially be granted citizenship.
Explanation: Answer is C