Answer:
At the boundary with the core.
Explanation:
The mantle is the second layer from the top. It is bordered by the lithosphere above it, or rather by the crust, and by the outer core below it. The mantle is a layer which is in viscous state, being mostly consisted of molten metals and rocks. The temperature of this layer is not the same all throughout it, but it varies a lot. The temperature at its top part is around 200 C degrees, while the temperature at its bottom is around 4,000 C degrees. There's two reasons for this. One is that the core is very hot, so it makes the bottom part of the mantle very hot as well, and the other one is that the lithosphere is much cooler, as well as constantly getting new, cold, crust in it, which makes its top part much cooler. The mantle is the layer responsible for the movement of the tectonic plates on the surface, as it is the layer where the convection currents occur and drive the plates' movement.
