Why Are Some Moons Such as Io and Enceladus Geologically

Why are some moons such as Io and Enceladus geologically active even though they are small in size compared to the planets?

A) Unlike some planets, these moons have additional supplies of radioactive elements providing the necessary heating to drive geological activity.
B) The interiors of these moons contain a larger supply of heavy elements such as iron than is found in terrestrial planets, which contributes to greater heating and high geological activity.
C) Tidal forces from the Sun are especially large for these moons, leading to greater interior heating and more geological activity.
D) Tidal forces from the parent planet heat the interiors of these moons, resulting in geological activity.
E) None of these; it is still a mystery.