Which agency in the U.S. is primarily responsible for regulating emissions from automobiles?

The Environmental Protection Agency (EPA) is the primary agency responsible for regulating emissions from automobiles in the United States. This includes setting and enforcing standards for air pollutants from both new and existing vehicles, as mandated by the Clean Air Act. The EPA's authority ensures that vehicles meet specific emission standards for various pollutants like carbon monoxide, nitrogen oxides, and particulate matter, thereby playing a crucial role in protecting air quality and public health.

The National Highway Traffic Safety Administration (NHTSA) focuses on vehicle safety standards and traffic regulations rather than emissions. The Department of Energy (DOE) is involved in energy policy and research related to energy efficiency, including some aspects of vehicle technology, but it does not regulate emissions directly. The Automotive Safety Agency (ASA) does not exist as an official agency; thus, it does not play any role in regulating emissions. Therefore, the EPA stands out as the correct answer due to its specific mandate concerning vehicle emissions regulation.