What was the first major step from hot molten early Earth toward a solid planet?

Correct answer: Cooling and solidifying of the outer surface

As the early Earth lost heat, its outermost material cooled and solidified, forming the first crust. This was the initial major step from a largely hot, molten body toward the differentiated solid planet that exists today. Option D is correct because surface cooling created a stable solid exterior, even though the deeper interior remained very hot and partly molten for a long time. Human agriculture, modern countries and books appeared billions of years later and are cultural developments, not stages in planetary formation. The key sequence is cooling, solidification and crust formation, followed by later atmospheric and hydrological changes.