Which layer of the Earth is primarily composed of molten rock?

The layer of the Earth that is primarily composed of molten rock is the mantle. The mantle lies between the crust and the outer core and is made up of silicate minerals that can exist in a semi-solid state. This semi-solid nature is crucial as it allows for the movement of tectonic plates due to convection currents within the mantle, which is generated by heat from the Earth's core.

While the outer core is also a layer involving molten material, it is primarily composed of liquid iron and nickel rather than rock. The inner core is solid, formed mostly of iron and nickel under immense pressure. The crust is the outermost layer of the Earth, composed of solid rocks and minerals but is not molten. Thus, the mantle is correctly identified as the layer primarily consisting of molten rock.