Final answer:
The use of leaded gasoline directly violates the Clean Air Act as lead is one of the six common air pollutants regulated by the EPA. While other activities listed also impact air quality, it is the explicit regulation of lead that makes the use of leaded gasoline a clear violation.
Step-by-step explanation:
Of the options listed, the use of leaded gasoline is a violation of the Clean Air Act. This is because the Clean Air Act includes provisions to reduce air pollution from a wide range of sources, including mobile sources such as vehicles. Burning fossil fuels, emissions of greenhouse gases, and disposal of hazardous waste could potentially violate the Clean Air Act if they result in pollution levels that exceed legal limits; however, it is the use of leaded gasoline that is explicitly regulated. The Environmental Protection Agency (EPA) has established air quality standards for six common air pollutants, which include lead. The combustion of leaded gasoline in piston engine aircraft, for instance, is a direct source of this pollutant.
It should be noted that while burning fossil fuels and emitting greenhouse gases do have environmental impacts and contribute to air pollution, their regulation under the Clean Air Act can be complex and depends on the specific circumstances and emissions involved.