Which layer of the Earth is solid but behaves like a liquid?

The asthenosphere is the correct answer because it is a layer of the Earth's upper mantle that, while solid, can flow very slowly due to its somewhat molten composition. This unique property allows it to behave like a viscous fluid over geological timescales. The temperatures and pressures at this depth are such that the rocks are not completely rigid; instead, they can deform and move, which is crucial for tectonic processes such as plate tectonics.

In contrast, the crust is the outermost layer, which is solid and rigid, not exhibiting the fluid-like behavior of the asthenosphere. The outer core is primarily composed of molten iron and nickel, making it a liquid layer, while the inner core is solid due to extreme pressures, despite its high temperatures. Thus, the asthenosphere uniquely combines solid and fluid characteristics, enabling movements that drive tectonic activity on the Earth's surface.