Final answer:
The lithosphere is a rigid layer comprised of the Earth's crust and the uppermost mantle, characterized by its brittle behavior unlike the plastically behaving asthenosphere below. The correct answer is option b.
Step-by-step explanation:
The lithosphere is defined as b. a rigid layer of crustal and mantle material. In detail, the lithosphere includes both the Earth's crust and the uppermost portion of the mantle which behaves as a brittle, rigid solid. Unlike the lithosphere, the asthenosphere beneath it is characterized by its partially molten upper mantle material that behaves plastically and has the ability to flow. Plate tectonics are a fundamental aspect of geology because the movement of tectonic plates, which are a part of the lithosphere, influences numerous geological processes.