Final answer:
The asthenosphere is a partially molten layer beneath the lithosphere characterized by its ability to flow and deform plastically, distinct from the brittle behavior of the lithosphere above.
Step-by-step explanation:
The lithosphere and asthenosphere are regions within Earth's interior, differentiated on the basis of their mechanical properties. The lithosphere includes the crust and the uppermost part of the mantle, characterized by its brittle behavior and rigidity. This layer can break under stress, leading to phenomena such as earthquakes. On the other hand, the asthenosphere, found just beneath the lithosphere, is known for its plasticity and ability to flow. This is because it is partially molten and can deform without breaking. It behaves in a ductile manner and is much weaker compared to the lithosphere which is why the correct answer to the student's question would be option c: Is relatively weak and flows readily.
The asthenosphere is vital in the process of plate tectonics, as it allows the tectonic plates of the lithosphere to move on top of it. It is important to note that the density of the asthenosphere is significantly lower than that of the Earth's core, which contradicts option d of the student's question. The core's density is much higher due to the presence of heavy elements such as iron and nickel under extreme pressure.