Final answer:
The Asthenosphere is the mechanical layer of Earth with the most active convection currents.
Step-by-step explanation:
The mechanical layer of Earth that has the most active convection currents is the Asthenosphere. The asthenosphere is a part of the upper mantle and is located just below the lithosphere. It is a semi-fluid layer where the solid rock behaves like a plastic and can flow slowly over time. Convection currents in the asthenosphere are responsible for the movement of tectonic plates on the Earth's surface.
Learn more about Asthenosphere and convection currents