Question

What invention most directly made the creation of the automobile and the airplane possible?

Answer 1

I am trying here...I think the internal combustion engine.

Explanation:

I think that the automobile and in particular the airplane needed a compact, comparatively light and very efficient engine to be feasible.
The steam engine works ok but it is heavy (you need to carry around a lot of water) and I do not think is very efficient. It is ok for steam engines and trains but for small cars or light airplanes I think it is not good enough.
The internal combustion engine uses the hot gas (air) produced by burning fuel to move the pistons and so doesn't need to carry a lot of heavy and voluminous water.