State governments are not sub-units of the federal government, but instead each is and does not report in any constitutional way to the federal government. a. a confederacy c. dependent b. sovereign d. an oligarchy

We are looking here for something that is compatible with "does not report in any constitutional way to the federal government" - and the correct answer is b. sovereign.

However, although the states are meant to be sovereign, there is some discussion about this: they are not perfectly sovereign, and for example the recent decision of the Supreme Court overrode the solutions of single states with respect to marriage equality.