What does “isostasy” refer to?

Isostasy refers specifically to the concept of buoyancy in the Earth's crust. It is the principle that explains how the Earth's lithosphere (the rigid outer layer of the Earth) behaves like a floating object on the more plastic asthenosphere underneath.

The idea is that the lithosphere is in equilibrium, and its movement can be influenced by the addition or removal of weight on it, such as ice, sediment, or mountain ranges. When weight is added, the lithosphere sinks deeper, and when weight is removed, it rises. This floating property explains many geological processes, including the adjustment of land after glacial melting or erosion.

This principle is distinct from the other concepts listed. The movement of ocean water pertains to oceanic processes, the formation of mountain ranges involves tectonic activity but does not directly describe buoyancy, and subduction refers to one tectonic plate moving under another, which is unrelated to the equilibrium of the crust. Thus, the correct understanding of isostasy centers around the concept of floating properties of rock layers.