Final answer:
The most significant item installed in automobiles of the era was the internal combustion engine, developed by Karl Benz and later mass-produced by Henry Ford in the Model T. The advent of the assembly line made cars more affordable, spurring the growth of related industries and changing social patterns.
Step-by-step explanation:
The most significant item being installed in automobiles during the era described was the internal combustion engine, which was initially developed by Karl Benz and later perfected in mass production by Henry Ford with the Model T.
Furthermore, advancements in automotive technology continued to unfold, as indicated by the enforcement of the Clean Air Act in 1970 which required automakers to include catalytic converters in new vehicles to meet emissions standards.