Final answer:
Tidal heating is the primary mechanism responsible for the internal heat of Io, which drives its volcanic activity. The intense gravitational pull from Jupiter deforms Io and generates heat through internal friction.
Step-by-step explanation:
The mechanism most responsible for generating the internal heat of Io that drives its volcanic activity is tidal heating. The intense gravitational pull of Jupiter causes significant tidal forces on Io. These forces deform Io's shape, creating friction and thus heat within its interior. This process is similar to how a stress ball heats up when continuously squeezed. This tidal heating is responsible for Io's active volcanism, making it an exception to the typical cooling behavior of smaller celestial bodies like our Moon, which cool relatively quickly after their formation.
While radioactive decay does contribute to internal heating in celestial objects, in the case of Io, the tidal forces from Jupiter provide the primary source of heat. Other mechanisms, such as accretion, bombardment, and differentiation, play lesser roles in comparison to the significant effect of Jupiter's gravity on Io.