Final answer:
The boundary between the lithosphere and the asthenosphere is the division between the Earth's rigid, brittle upper mantle and the more malleable, plastic mantle below. The correct identification of this boundary is option (a), the boundary between rigid mantle and plastic mantle, and not the crust-mantle boundary, known as the Moho.
Step-by-step explanation:
The boundary between the lithosphere and the asthenosphere is the division between the uppermost rigid mantle and a more ductile, or 'plastic,' upper mantle that can flow. The lithosphere includes the crust and part of the upper mantle, both of which behave in a brittle, rigid manner. So the correct answer is (a) the boundary between rigid mantle and plastic mantle. However, it's also important to clarify that the Moho, or Mohorovičić discontinuity, is the boundary between the crust and the mantle and is not the same as the lithosphere-asthenosphere boundary.
The lithosphere is approximately 100 kilometers thick and includes the crust, but based on mechanical behavior, extends into the uppermost mantle. This layer is cool and rigid compared to the underlying asthenosphere, which behaves plastically due to higher temperatures just below the lithosphere. When the lithosphere is subjected to stress, it breaks, causing earthquakes. Therefore, the lithosphere-asthenosphere boundary is a critical component in the understanding of tectonic activities. The asthenosphere, on the other hand, allows for the slow convection of mantle material, which drives the movement of tectonic plates on the Earth's surface.