Final answer:
The average density of the Galilean moons increases and geological activity escalates from Callisto to Io due to the strong tidal heating from Jupiter's gravity, with Io being the most volcanically active body in the solar system.
Step-by-step explanation:
As we move inward from the Galilean moon Callisto to Io, we observe that the average moon density increases, and geological activity also increases. Callisto is a less differentiated and lower-density moon with a heavily cratered surface, indicating an old and less geologically active body.
In contrast, Io, being closer to Jupiter, experiences significant tidal heating due to Jupiter's gravity, making it the most volcanically active body in our solar system, with an internal heat source that sustains its active geology. Furthermore, Europa and Ganymede, moons that lie between Callisto and Io, have varied levels of geological activity and densities due to their respective distances from Jupiter and resulting tidal forces.