Which of the following best defines the lithosphere?

The lithosphere is best defined as the rigid outer layer of the Earth, which encompasses both the crust and the uppermost portion of the mantle. This layer is characterized by its solid and brittle properties, allowing it to break and form tectonic plates. These plates float and move on the underlying semi-fluid asthenosphere, which is responsible for their dynamic behavior. The lithosphere plays a critical role in tectonic processes, as its movements can lead to geological phenomena such as earthquakes, volcanic activity, and mountain formation. Understanding the lithosphere's composition and structure is essential for studying plate tectonics and the geological features shaped by tectonic activity.