What agreement ended the original system of direct popular votes for electors?

The Constitution is the correct answer as it established the framework for the electoral process in the United States, including the selection of the Electoral College. It originally called for electors to be chosen by state legislatures rather than through direct popular votes by the public. This system was designed to balance the influence of populous states with less populous ones in the election of the President. Over time, many states transitioned to using popular votes to select their electors, but the original framework for this process was set by the Constitution.

The other choices do not align with the historical context of altering the direct popular vote system for electors. The Federal Election Reform Act relates to election laws and regulations but does not specifically address the selection process of the Electoral College. The Great Compromise was focused on legislative structure rather than the electoral process itself, and the Voting Rights Act aimed to eliminate barriers to voting, particularly for racial minorities, but did not directly address the method of selecting electors as outlined in the Constitution.