Which of the following is NOT one of the three main layers of the Earth?

The three main layers of the Earth are the crust, the mantle, and the core. The crust is the outermost layer where we live, the mantle lies beneath the crust and consists of semi-solid rock that flows slowly, and the core is divided into a liquid outer core and a solid inner core, primarily made of iron and nickel.

The asthenosphere, while an important part of Earth's structure, is not one of the three main layers. Instead, it refers to a region within the upper mantle characterized by its semi-fluid properties that allow tectonic plates to move on top of it. Since the question asks for a layer that is not one of the fundamental main layers, the asthenosphere fits this criterion as it is a sub-layer of the mantle rather than a distinct layer like the crust, mantle, or core.