Final answer:
The correct answer is option a. The Earth's continents and seafloors form the crust, consisting of oceanic and continental types with varying thicknesses and compositions of basalt and granite, respectively.
Step-by-step explanation:
The Earth's continents and seafloors form a thin outer layer known as the crust. This layer is composed of two main types of crust: the oceanic crust and the continental crust. The oceanic crust, predominantly made of basalt, is approximately 6 kilometers thick and covers 55% of Earth's surface, lying mainly under the oceans.
On the other hand, the continental crust, composed mainly of granite, is significantly thicker, ranging from 20 to 70 kilometers, and covers 45% of the Earth's surface, with some sections also beneath oceans. These crustal rocks have similar densities of about 3 g/cm³ and are the layers most accessible for geological study, despite representing only 0.3% of Earth's total mass.