What layer of the Earth do tectonic plates float upon?

Tectonic plates float upon the asthenosphere, which is located just below the lithosphere. The asthenosphere is part of the upper mantle of the Earth and exhibits plasticity, allowing it to flow slowly over geological time. This characteristic enables the rigid tectonic plates of the lithosphere to move on its surface. The movement and interactions of these plates are integral to various geological processes, including earthquakes, volcanic activity, and the formation of mountain ranges. Thus, the asthenosphere provides a semi-fluid layer that supports the tectonic plates, making it the correct answer in this context.