Explanation
Correct answer: D.
Choice D is the best answer. The claim is that divided Congresses are necessary but insufficient—that is, we need divide Congresses, but they are not enough—to decrease government size, as measured by total federal outlays. This choice accurately expresses the supporting data from the "change in total outlays" part of the graph. Within the data set, divided Congresses sometimes decreased total outlays, but undivided ones never did.
Why the other choices are wrong
Choice A
Choice A is incorrect. The claim is only about government size, as measured by total federal outlays—defense and nondefense outlays aren't relevant.
Choice B
Choice B is incorrect. The claim is only about government size as measured by total federal outlays—nondefense outlays aren't relevant.
Choice C
Choice C is incorrect. The claim is only about government size as measured by total federal outlays—specific information about defense or nondefense outlays isn't relevant.