Which option can be a source of internal heat?

Correct answer: Decay of radioactive materials

Earth’s interior remains hot because heat comes from more than one source. One important source is radioactive decay: unstable isotopes inside Earth gradually break down and release energy as heat. This heat contributes to the thermal energy of the mantle and supports the long-term internal processes associated with convection and plate movement.

Option A is therefore the scientifically relevant choice. A school bell is a sound-producing object, a river boat uses external energy for movement, and a farming seed stores biological energy for growth; none is a source of Earth’s internal heat. Radioactive decay does not mean a sudden fire, but a slow, continuing release of heat within rocks and minerals.