LP gas portable cylinders must be designed, fabricated, tested, and marked in accordance with which agency's regulations?

LP gas portable cylinders must be designed, fabricated, tested, and marked in accordance with the regulations established by the US Department of Transportation (DOT). This is because the DOT is responsible for the safety and regulation of all modes of transportation, which includes the transportation of hazardous materials such as liquefied petroleum gas (LP gas). The materials, construction, and safety features of these cylinders must comply with DOT standards to ensure they are safe for transport and use.

The DOT regulations encompass specifications for cylinder design, such as pressure ratings, material strength, and the testing processes to ensure that they can safely withstand the conditions they will encounter during storage and transport. These regulations are crucial for maintaining public safety and preventing accidents related to the use of LP gas.

The other agencies mentioned, while involved in various aspects of safety and regulation, do not specifically govern the design and testing of LP gas cylinders. For example, the Environmental Protection Agency deals primarily with environmental protection regulations, the Federal Aviation Administration focuses on aviation safety, and the Occupational Safety and Health Administration oversees workplace safety regulations.