Which energy source contributes the most to heating the ground?
a. friction
b. solar radiation
c. radioactive decay
d. the differential rotation of earth's core

Solar Radiation, aka sunlight. The sun is the primary source of heat for the planet. Without it, we would freeze at the surface. Friction is negligible since we don't create near enough, if there was radioactive decay heating up the surface more than the sun, we would be dead, and the last one is just a teaser.