Which layer of the Earth lies beneath the lithosphere and is part of the upper mantle?

The asthenosphere is the layer of the Earth that lies directly beneath the lithosphere and is part of the upper mantle. This layer is crucial as it is composed of semi-solid rock that can flow slowly over time, allowing tectonic plates in the lithosphere—floating above it—to move. The asthenosphere's properties make it essential for the mechanism of plate tectonics, as it provides the necessary support for the plates to move without being overly rigid.

The crust, while being the outermost layer, does not lie beneath the lithosphere but rather constitutes the lithosphere itself. The mesosphere refers to the lower mantle, which exists at greater depths than the asthenosphere. The core, meanwhile, is a separate layer composed mainly of iron and nickel, located at the center of the Earth, far beneath both the mantle and the lithosphere. Thus, the asthenosphere is correctly identified as the layer that lies beneath the lithosphere and forms part of the upper mantle, making it the appropriate answer.