Which of the following prevents the enactment of ex post facto laws?

The correct choice is Article I of the Constitution, which explicitly prohibits the enactment of ex post facto laws. This prohibition is found in both Article I, Section 9, which restricts Congress from passing such laws, and Article I, Section 10, which prevents states from doing the same. An ex post facto law is one that retroactively changes the legal consequences of actions that were committed before the law was enacted, often undermining the principle of legality and fairness in the legal system.

The Due Process Clause focuses more broadly on ensuring that all individuals receive fair treatment under the law but does not specifically address ex post facto laws. The Bill of Rights comprises the first ten amendments to the Constitution, outlining various personal freedoms and rights, but it, too, does not contain a specific prohibition against ex post facto laws. The Separation of Powers refers to the distribution of government responsibilities among different branches to prevent any one branch from exercising too much power; it does not directly relate to the prohibition of ex post facto legislation. Thus, Article I is the clear and specific source for this prohibition in the Constitution.