the earth is composed of multiple layers, which can be defined either by composition or by mechanical properties. this layer is key to plate movement. it is a solid but flows like a very viscous fluid. it is the
a) mesosphere.
b) lithosphere.
c) lower mantle.
d) asthenosphere.

Option (D)

Explanation:

The asthenosphere is the layer that is present in the upper portion of the mantle and is below the lithosphere at a depth from 80 to 200 km below the earth's surface. The rocks in this layer are solid in nature and behaves in a plastic manner. They have the ability to flow in response to the deformation.

The convection current forms in the mantle due to the heat energy radiated from the interior of the earth. These currents become more effective in the layer of asthenosphere that forces the lithospheric plates to move from one place to another.

Hence, the correct answer is option (D).