Answer:
the orbit of Io around Jupiter causes the tidal heating.
Explanation:
Io's elliptical orbit causes the tidal force to vary as it orbits Jupiter.
Because Jupiter is a very massive planet, it has a very large gravitational pull on its moons, so the side of Io facing the planet is experiencing a greater gravitational pull than the opposite side. This causes a distortion in the shape of the Io, and a friction that causes the tidal heating.
This friction generates heat currents, which is what makes Io the place with the most volcanic activity in the solar system.
