Io maintains a constant orbit around Jupiter because
A
Io is held trapped within Jupiter's atmosphere due to friction caused by air resistance.

B
Io has been caught in the magnetic field created by Jupiter's rotation about its own axis.

C
the gravitational pull from Jupiter is much greater than any other forces acting on Io.

D
high pressures and gases cause reactions on Jupiter that pull Io toward the planet.

C . The gravitational pull from Jupiter is much greater than any other forces acting on Io.

Explanation:

Let's begin by the fact Jupiter is a huge massive planet, therefore the bodies that are closest to Jupiter will have a greater gravitational force than the other bodies. That is the situation with Io, the moon closest to Jupiter.

However, Io's orbit is very elliptical and eccentric compared to the outer moons, resulting in the distortion of its shape by the gravitational force.  In fact, Io is considered the most geologically active object in the solar system, due to these variations.

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