Since D is literally saying that it prevents the government from obligating citizens to defend the country, I'd say that that may not be the best answer.
For C, it could be right (as you're forced to be on a jury), but we need to make sure that it's the best answer
For B, taxes defends the nation (as a nation cannot run without taxes), so it could be right.
For A, when the country is in way or another emergency, citizens are obligated to quite literally defend the country and join the war effort. Since B and C simply play a role in benefiting the US, but this is more direct (and arguably helps more than paying a few thousand dollars in taxes), I'd say that A is the right answer here. Without citizens to be on juries or paying taxes, the government could still function for a certain amount of time, but without troops, our country is essentially screwed
