Final answer:
The most important source of energy for heating the interiors of terrestrial planets today is radioactivity, particularly from the decay of isotopes such as uranium, thorium, and potassium. So, the correct option is c) radioactivity.
Step-by-step explanation:
The most important energy source in continuing to heat the interiors of terrestrial planets today is c) radioactivity. The reason why radioactivity plays such a crucial role is due to the decay of radioactive isotopes such as uranium, thorium, and potassium.
Geologists estimate that about half of Earth's current internal heat budget comes from the decay of these radioactive isotopes. This heat is necessary for geological activity and gives insight into the energy sources driving a planet's geothermal energy.
Other sources of internal heat, like accretion and differentiation, played a role early in the formation of a planet, but their contribution has since diminished. Tidal heating is significant for celestial bodies like Jupiter's moon Io, influenced by the strong gravitational pull of Jupiter, but it is not a general source of heat for all terrestrial planets. The greenhouse effect is a process that warms the surface and atmosphere of a planet rather than its interior.