Question

what is the main source of the heat that drives convection currents inside the earths mantle?
friction from plate movement
lightning strikes
sunlight
radioactive decay of unstable isotopes

Explanation:

The heat driving convection currents inside the Earth's mantle mainly comes from the radioactive decay of unstable isotopes. Plate movement friction is a minor factor. Lightning strikes occur in the atmosphere and don't contribute to mantle heat. Sunlight heats the Earth's surface, not the mantle's interior heat for convection.

Answer:

Radioactive decay of unstable isotopes.