Final answer:
Delta Air Lines started replacing propeller aircraft with jet aircraft in the late 1950s, marking the advent of a more efficient era in aviation.
Step-by-step explanation:
Delta Air Lines began replacing its propeller aircraft with jet aircraft in the late 1950s, with the introduction of commercial jetliners. This innovation significantly enhanced the efficiency of air travel. The development of jet engine technology saw rapid gains after World War Two, with jet fighters being used during the Korean War and commercial jets entering the airspace in the 1950s. This shift led to the eventual phasing out of ocean liners by the 1970s for passenger transport, marking a new era in aviation history.