Final answer:
Option D ('Under the Articles, Congress was allowed to regulate trade') is not a reason for the weakened post-Revolutionary War American economy because actually, Congress could not regulate trade under the Articles, leading to economic discord.
Step-by-step explanation:
The reason that is NOT why the Articles of Confederation weakened the post-Revolutionary War American economy is option D: 'Under the Articles, Congress was allowed to regulate trade.' In reality, the Articles did not allow Congress to regulate trade, leading to economic conflicts and issues between states due to the lack of a unified commercial policy. Therefore, all the other options indicate real weaknesses caused by the Articles: A) Only states had the power to tax, which prevented the federal government from paying off war debts. B) Congress had no resources to pay the military. C) Both states and Congress had the rights to issue money, causing economic problems due to inconsistent currency.