The correct answer is D.
It is possible to be a U.S. national but not a citizen. On the contrary, all U.S. citizens are U.S. nationals as well.
A U.S. national owns his sole allegiance to the United States.
People who were born in American Samoa and Swains Island are considered U.S. nationals.
Also U.S. nationals are people born in the Commonwealth of Northern Mariana Islands.
U.S. nationals don't have the same rights as U.S. citizens but are under protection of the U.S. government.