How does obligating citizens to defend the nation benefit the united states?

a- it ensures that the country will be protected in times of war or extreme national emergency
b- it allows the government to collect revenue from its citizens
c- it guarantees that the nations judicial system remains fair and impartial
d- it prevents the government from instituting a national draft

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