Which claim is not defensible?

A. The nation's debt generally rises during times of war.

B. Decreasing taxes on middle-class citizens improves the U. S.
economy.
C. The world would be better off if political boundaries didn't exist.
D. The government works better when the president and the majority
of Congress are from the same party.

All of these are defensible. Of course debt rises in war, and decreasing taxes will benefit an economy where taxes are no longer needed (post-scarcity.) Political and geographical boundaries are outmoded and a world without them is not only possible but existed for much of early human civilization. As for the government, a government would run more efficiently when everyone is in basic agreement with what to do and how.

I would question your teacher on this. Anyone can defend these perspectives...