Which is the most volcanically active celestial body in solar system ?

 This question based on astronomy was asked on Koun Banega Karodpati program recently.

Question 

Which is the most volcanically active celestial body in solar system ?

1.  A star

2.  A planet 

3.  A natural satellite 

4.  An asteroid 

Answer 

I knowp that tallest volcano in our solar system is in planet Mars, which is Olympus Mons.

But is it active or not I don't know.

So, we can eliminate the option planet.

I never heard of a volcano on an asteroid.

No one explored a Star till now  !!!!

So we are left with natural satellite.

It is the correct answer  !!!!!

Io, which is one of the moons of planet Jupiter is the celestial body with most volcanic action.

This intense volcanic activities are due to Jupiter's gravitational pull which creates internal heat and tectonial activities within the satellite Io.

This results in hundreds of volcanoes and a surface covered with lava  flows and volcanic debris.