What Is the Kelvin-Helmholtz Mechanism

What is the Kelvin-Helmholtz mechanism?

A) the tidal flexing which causes Io to be geologically active
B) a satellite in a non-equatorial orbit will have its orbit gradually shifted toward the equatorial plane
C) the release of gravitational potential energy as heat when a body contracts
D) the shift of the spectral peak to longer wavelengths as a body cools off