Which part of Earth is known as the rigid portion consisting of the crust and upper mantle?

The lithosphere is the rigid outer layer of Earth, composed of the crust and the uppermost part of the mantle. This layer is characterized by its solid state, which allows it to maintain structural integrity and stability. It is divided into tectonic plates that float on the more pliable asthenosphere beneath it. The movements and interactions of these plates are central to the theory of plate tectonics, leading to phenomena such as earthquakes, mountain building, and volcanic activity.

The other options represent different layers or components of the Earth. The asthenosphere, for instance, is a semi-fluid layer beneath the lithosphere that allows for the movement of tectonic plates. The core refers to the innermost layer of the Earth, which is comprised of a solid inner core and a liquid outer core. The mesosphere, or lower mantle, is situated beneath the asthenosphere and is also solid but behaves differently under pressure compared to the lithosphere. Understanding these distinctions is crucial in the study of Earth's structure and tectonic processes.