What was the most direct cause of the formation of Earth's outer solid layer?

Correct answer: Cooling and solidification of the surface

The governing process was cooling and solidification during the early evolution of Earth. The newly formed planet initially contained abundant heat from accretion, compression, radioactive decay, and impacts, so much of its outer material was molten or semi-molten. As the surface lost heat to space, it cooled and hardened into the first solid crust. Hence option A is the only scientifically appropriate answer. Atmospheric disappearance is not the direct mechanism, and farming occurred billions of years later. Ozone damage is a modern atmospheric issue and has no role in creating the original rocky outer layer.