The principle of isostasy refers to:

A) the difference in the strength of the mantle versus the crust
B) the relationship between regional elevations and thickness of crust
C) how the outer core differs from the inner core
D) how the upper mantle differs from the lower mantle

The principle of isostasy refers to the equilibrium that exists between parts of the Earth's crust, which is maintained by the flow of materials in the Earth's mantle. This principle explains how the Earth's crust floats on the denser, underlying mantle and how regional elevations are related to the thickness of the crust. Essentially, areas of thicker crust, such as mountain ranges, are buoyed up by the mantle, whereas thinner areas, such as ocean basins, sit lower. This concept is crucial in understanding the distribution of topographic heights on Earth.