Which of the following is required to be installed on an airplane manufactured after July 18, 1978?

The requirement for a shoulder harness for each front seat on airplanes manufactured after July 18, 1978, is mandated by the Federal Aviation Administration (FAA) under the regulations that enhance passenger safety. This regulation aligns with advancements in aviation safety standards, specifically focusing on reducing neck and head injuries during turbulence or accidents. The incorporation of shoulder harnesses represents a significant improvement over earlier safety measures and aligns with the goal of ensuring the highest level of occupant safety within the cockpit area.

Regarding the other potential options, while emergency locator beacons, equipment lists, and fuel flow gauges may have their own respective requirements or benefits, they do not have the same regulatory mandate concerning the installation of shoulder harnesses for front seats on aircraft built post-1978.