Which aviation act marked the beginning of the unified regulation of commercial aviation in the U.S.?

The Air Commerce Act of 1926 is significant because it established a framework for the regulation of civil aviation in the United States. This act initiated the federal government's involvement in the safety and promotion of air travel and set the groundwork for what would become a more comprehensive regulatory system for commercial aviation.

Through this legislation, the government began to define critical aspects of aviation, including air traffic rules, licensing of pilots, and certification of aircraft. By recognizing aviation as a critical mode of transportation that required oversight, this act marked a pivotal moment in the evolution of the aviation industry in the U.S.

The other choices focus on different aspects of aviation history but do not represent the same level of regulatory unification. The Air Mail Acts primarily dealt with the postal service aspects of aviation rather than laying out an overarching regulatory framework, which is why they do not align with the question regarding unified regulation of commercial aviation.