Which legislative action would most likely violate the Supremacy Clause?

The Supremacy Clause of the United States Constitution establishes that federal law takes precedence over state laws and constitutions. This means that if there is a conflict between federal and state legislation, federal law will override state law.

The Occupational Safety and Health Act (OSHA) is a federal law designed to ensure workplace safety and health regulations across the country. If a state were to enact a law that conflicts with OSHA regulations, such as setting less strict safety standards, this would likely violate the Supremacy Clause. In this context, OSHA's federal standards would take priority over any conflicting state legislation, thereby illustrating the principle behind the Supremacy Clause.

The other options provided reflect federal laws that generally do not have widespread conflicts with state laws, as they pertain to specific areas of regulation (healthcare, aviation, and forestry). Even though these laws could potentially encounter state-level regulations, they are not typically associated with direct conflicts that would arise in the same way as OSHA laws regarding workplace safety might. In this sense, OSHA stands out as the legislative action most likely to run afoul of the Supremacy Clause due to the nature of its enforcement and the potential for contradictory state laws.