What act marked the beginning of the U.S. government's responsibility for economic affairs?

The Interstate Commerce Act of 1887 is significant as it marked a foundational moment in the U.S. government's responsibility for regulating economic affairs, particularly in the realm of transportation. This Act was designed to regulate the railroad industry and its monopolistic practices, which were seen as detrimental to fair competition and consumer protection.

By establishing the Interstate Commerce Commission (ICC), the government took a proactive role in overseeing the rates and practices of railroads, ensuring they were fair and just to all stakeholders, including farmers and other small business operators. This legislation represented the first federal effort to regulate an industry, highlighting a shift toward government intervention in the economy, which laid the groundwork for future regulatory measures in various sectors.

In contrast, the other options, while important, pertain to specific financial or economic issues, such as securities regulation, monetary policy, and banking reforms, which do not mark the initial responsibility of the government in economic affairs as effectively as the Interstate Commerce Act does.