Final answer:
The lithosphere includes both the crust and the uppermost mantle, which are mechanically rigid, distinguishing it from the more plastic asthenosphere beneath.
Step-by-step explanation:
The lithosphere is a layer of Earth that includes both the crust and the upper part of the mantle. Specifically, it consists of Earth's outer surface, the crust, which is a thin, brittle shell made of rock, and the uppermost part of the mantle that behaves as a rigid, solid layer. Hence, the correct answer to the question is that the lithosphere consists of the uppermost mantle and the crust. This definition is based on the mechanical properties of these Earth layers, distinguishing it from the asthenosphere, which lies beneath the lithosphere and behaves plastically, allowing for the flow of mantle materials and facilitating tectonic plate movements.