Which claim is not defensible?
a. decreasing taxes on middle class citizens improves the u. s. economy.
b. the government works better when the president and the majority of congress are from the same party.
c. the nations debt generally rises during times of war.
d. the world would be better off if political boundaries didn't exist.

"The world would be better off if political boundaries didn't exist" is an indefensible claim. The others can be proven or at least debated over because at some point in time all of them have true and false. For instance, the majority of congress and the President have been from the same political party. Also, the majority of congress and the President have NOT been from the same political party. Therefore, you can compare the difference between true and false. Political boundaries have always existed. As a result, you cannot comfortably compare it to anything else.

The principle that george w. bush used to keep white house communications confidential would be executive privilege. but i maybe wrong : )