Which agency regulates all piping from the gas main up to the outlet side of the gas meter?

The correct choice is the Department of Transportation's Pipeline and Hazardous Materials Safety Administration (DOT PHMSA). This agency plays a crucial role in ensuring the safety and reliability of the nation’s pipeline systems, which includes the regulation of gas piping from the gas main to the outlet side of the gas meter. PHMSA sets safety standards, conducts inspections, and oversees the implementation of safety regulations to protect the public and the environment from the hazards associated with gas transmission and distribution.

The other agencies mentioned, such as the Environmental Protection Agency and the National Fire Protection Association, focus on different areas of public safety and environmental protection. The Environmental Protection Agency primarily deals with environmental regulations and protection measures, while the National Fire Protection Association promotes fire safety codes and standards, but does not regulate gas piping directly. The Department of Transportation does oversee various aspects of transportation including pipelines, but the specific regulatory responsibilities for gas piping fall under PHMSA. Therefore, understanding the specific role of each agency clarifies why DOT PHMSA is the correct answer regarding regulation of gas piping safety from the main to the meter.