Which combination is most correct for the heating of early Earth?

Correct answer: Collisions, gravitational compression, and radioactive decay

The early Earth became hot through several interacting processes during its formation. Repeated collisions among planetesimals converted kinetic energy into heat, and the growing body was compressed by its own gravity, producing additional internal heating. Radioactive elements also released heat as they decayed. These sources helped create a hot, partly molten Earth, which later allowed heavier materials such as iron to sink and lighter materials to rise during planetary differentiation. Option D is correct because it includes the major mechanisms together. Cold rain and snowfall cannot explain the initial heating, present sunlight is not the only or principal internal source, and ocean waves are far too weak to heat a planet-sized body significantly.