Final answer:
The bottom layer of the Earth's crust is composed of heavier rocks such as a. granite and basalt, both of which are igneous rocks formed from cooled lava and are rich in silicon and oxygen alongside other elements.
Step-by-step explanation:
The composition of Earth's crust, particularly the bottom layer, is characterized by heavier rocks. The correct answer is a) Granite and basalt, with granite being more prevalent in continental crust and basalt dominating oceanic crust. Both types of rocks are classified as igneous, signifying they formed from the cooling of molten material.
Basalt is rich in elements like silicon, oxygen, iron, aluminum, and magnesium, and is the main constituent of the oceanic crust. Continental crust, on the other hand, is primarily composed of granite, which includes the same elements but with different proportion.