Which organization must approve the specifications for high-pressure oxygen cylinders installed in airplanes?

The correct answer is that the specifications for high-pressure oxygen cylinders installed in airplanes must be approved by the Department of Transportation (DOT). The DOT is responsible for the regulation of the transportation and safety of hazardous materials, including compressed gases like oxygen. This organization establishes standards for the design, testing, and maintenance of high-pressure cylinders to ensure that they are safe for use in aviation.

Oxygen systems are critical for flight safety, especially at high altitudes where the atmosphere is thin. Therefore, the DOT's rigorous standards help to minimize the risks associated with handling and storing oxygen in a high-pressure environment.

The roles of the other organizations mentioned differ from that of the DOT. The Federal Aviation Administration (FAA) primarily governs the safety and airworthiness of aircraft and their components, while the Environmental Protection Agency (EPA) focuses on environmental regulations rather than the specifics of transportation and cylinder specifications. The National Transportation Safety Board (NTSB) investigates transportation accidents and makes safety recommendations, but does not have a role in the approval of cylinder specifications. Thus, the DOT is the appropriate authority for this matter.