The correct answer is:
The invention of passenger elevators.
Step-by-step explanation:
The invention of passenger elevators was the greatest factor in the development of skyscrapers (very tall commercial buildings). The first skyscraper was the Home Insurance Building (originally made of 10 stories) in Chicago, and designed by William Le Baron Jenney. Skyscrapers became more frequent and each time taller because they allowed to create more stories (floors) and to make the most out of a limited space, mainly because passenger elevators made possible to translate from floor to floor without using the stairs.