Concept:The Earth's crust is classified into layers based on the proportions of silica (SiO2), alumina (Al2O3), and magnesium.The name "Sial" is derived from silica and alumina.Explanation:The outermost layer of the crust is called Sial.It consists mainly of silica (SiO2) and alumina (Al2O3).Sial is lighter and forms the continental crust.Below Sial lies Sima, which is rich in silica and magnesium.Since Sima does not contain alumina, the layer made of silica and alumina must be Sial.Answer:D. Sial