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

The correct answer is the Federal Aviation Administration (FAA). The FAA is the primary regulatory body overseeing civil aviation in the United States, and it has the authority to set standards and approve specifications for equipment used in aircraft, including high-pressure oxygen cylinders.

These cylinders must comply with stringent safety and performance standards to ensure they can withstand the demands of aviation, especially in terms of pressure and environmental conditions. The FAA also oversees the installation, maintenance, and operation of these systems to ensure they meet safety regulations.

While the Environmental Protection Agency (EPA) and the Department of Transportation (DOT) have roles in regulating aspects of transportation and environmental impact, the specific control of aviation-related safety standards, including oxygen systems, falls within the FAA's jurisdiction. The National Safety Board focuses more on investigating accidents rather than regulating specification approvals.