The earth is composed of three large layers: the nucleus, the mantle, and the crust. However, there are more layers within the main layers; in the case of the the asthenosphere, this is a layer of semi-molten materials that is located under the lithosphere (50 to 150km of depth) and has a thickness of around 650km. Additionally, this is located above the mesosphere, another layer of the crust that is part of the mantle, and extends to approximately 2700km deep. Additionally, this layer serves as a transition between the lithosphere and the mesosphere which are two major layers. So, the correct answer is A. Asthenosphere.
