Which claim is arguable?

a. the department of defense is the largest department in the federal government.
b. ronald reagan was an actor before becoming president of the united states.
c. the supreme court is the highest court in the u. s. judicial system.
d. voting is the best way to make a difference in politics.

"arguable" means that the statement's claim can be debated. It's possible to prove it right or prove it wrong. In other words, it's not 100% clear what the true answer is. It's more like an opinion if anything. Saying something like "Ronald Reagan was an actor" is very clear which means it's not arguable.

However, saying something like "Voting is the best way to make a difference in politics." is arguable because there may be other avenues to make a difference. This point can be debated with facts and figures.

So that's why D is the best answer. Everything else are hard facts that can't be changed no matter how persuasive the argument.