Final answer:
Heat travels in slow convection currents of solid materials within Earth's mantle due to the decay of Earth's radioactive isotopes.
Step-by-step explanation:
Heat travels in slow convection currents within Earth's mantle. These currents are a by-product of the decay of Earth's radioactive isotopes, known as geothermal heat. Therefore, the correct answer is A. is a by-product from the decay of Earth's radioactive isotopes.
Learn more about Convection currents in Earth's mantle