Which structure first developed when Earth's outer surface solidified?

Correct answer: Crust

The crust first developed as Earth’s hot outer material cooled and solidified. In the early history of the planet, intense heat kept much of the surface molten or semi-molten. As cooling progressed, the outermost material became solid and formed a relatively thin rocky layer called the crust. This was an important stage in Earth’s physical evolution and later allowed the development of oceans, atmosphere and life-supporting environments. Dense forests and permanent cities appeared much later, after biological and human evolution. A glacier is a mass of ice formed under particular climatic conditions, not the first solid structure produced by Earth’s cooling. Hence, option D is correct.