Final answer:
Isostasy refers to the equilibrium in buoyancy between the lithosphere and the asthenosphere, critical for understanding the Earth's crust's vertical movements in response to mass distribution and tectonic activities.
Step-by-step explanation:
The correct answer to the provided question is b) Isostasy. Isostasy refers to the equilibrium in buoyancy between the lithosphere, which includes Earth's crust and the upper rigid portion of the mantle, and the underlying semi-fluid asthenosphere. This geological principle explains how the Earth's crust maintains its level due to the balance of mass and buoyant forces. When material is added or removed from the crust, such as during mountain building or erosion, the lithosphere will rise or sink until balance is reestablished. This is crucial for understanding how the Earth's crust behaves in response to different geological processes and is intimately related to the theory of plate tectonics.
Plate tectonics, on the other hand, explains the movement of the Earth's lithospheric plates and the interactions at their boundaries. This fundamental geologic theory involves the movement of large segments of the crust due to underlying mantle convection, resulting in geological phenomena such as earthquakes, volcanic activity, and the drifting of continents.
While the concept of isostasy does not describe the motion of the plates themselves, it is an essential concept for understanding the vertical motions of the lithosphere in response to changes in mass distribution resulting from tectonic activities governed by the principles of plate tectonics.