What is the geological layer between the Earth's crust and core?

The correct choice is the Mantle, which is the geological layer situated between the Earth's crust and the outer core. The mantle is composed of silicate minerals that are rich in iron and magnesium and is characterized by its semi-solid state, allowing it to flow slowly over geological timescales. This layer plays a crucial role in tectonic activities, as convection currents in the mantle are responsible for the movement of tectonic plates located on the Earth's crust.

The upper mantle forms part of this layer, but it does not encompass the full range of materials and characteristics attributed to the entire mantle. The lower and upper crust mistakenly represent divisions within the crust itself rather than the layer that exists between the crust and the core. Therefore, the mantle is the most appropriate term when referring to the layer bridging the crust and the core.