Which layer acts as the soft part of the mantle that allows tectonic plates to move?

The asthenosphere is the correct answer because it is a region of the upper mantle that is characterized by its semi-fluid state, allowing it to behave like a viscous material. This softness is crucial for enabling the movement of tectonic plates, which are part of the lithosphere above. The asthenosphere extends from about 100 to 700 kilometers beneath the Earth's surface and provides an underlying layer that accommodates the motions of the tectonic plates as they float and shift due to convection currents in the mantle.

In contrast, the lithosphere is rigid and encompasses both the crust and the uppermost part of the mantle, which does not allow for the mobility of plates. The lower mantle, located beneath the asthenosphere, is much more solid due to the immense pressure and temperature, preventing the kind of movement necessary for tectonic activity. The upper mantle includes both the lithosphere and asthenosphere, so it doesn't specifically refer to the soft part responsible for plate movement like the asthenosphere does. Therefore, the asthenosphere is specifically recognized for facilitating the dynamic nature of tectonic plates.