Which agency is primarily responsible for enforcing the Clean Air Act?

The agency primarily responsible for enforcing the Clean Air Act is the Environmental Protection Agency (EPA). The Clean Air Act, enacted in 1970 and amended several times, is a significant piece of environmental legislation aimed at regulating air emissions from stationary and mobile sources. The EPA oversees the implementation of this act by setting national air quality standards that are enforced across the country.

The EPA has the authority to enforce compliance, conduct inspections, and impose penalties for violations of the Act. It works with state and local agencies to develop and enforce air quality standards, thus ensuring that the standards are met effectively. While state environmental agencies and local governments play a role in implementation and enforcement at their respective levels, the EPA is the federal entity that oversees the overall enforcement and compliance with the Clean Air Act. The Department of Energy, while significant in energy policy and related environmental matters, does not have direct enforcement authority over air quality under the Clean Air Act. Thus, the EPA stands out as the correct answer due to its primary role in this regulatory framework.