How did airmail service influence the development of commercial passenger service?

The correct answer highlights that as airmail services expanded, they demonstrated the viability and potential profitability of air travel. The successful operation of airmail routes not only showcased the efficiency of airplanes in transporting goods quickly over long distances, but it also developed a framework that could be adapted for carrying passengers. Observing the demand for fast and reliable airmail service, airlines recognized that there was a market for passenger transport as well. This led to the introduction of commercial passenger flights, where passengers expressed a willingness to pay for the speed and convenience that air travel provided, thus solidifying the business case for expanding commercial aviation.

In contrast, while faster travel, speedier mail deliveries, and support for airmail initiatives were relevant considerations, they were more indirect influences compared to the direct relationship between the demonstrated success of airmail services and the willingness of passengers to pay for flights.