Which is the force of repulsion between two positively-charged particles?

 answer>>>>electric force <<<<by the way i don't like physics but i answer this for you ^-^

This is because centripetal force is just the net force of a circular motion. There are no attractive or repulsive forces here. This is not the case here. 
The gravitational force is a force reliant on mass and attraction of the masses. There are attractive forces here, but not really repulsive forces. 
The electric force is the only one that would make sense because it has to do with a relationship between charges and includes both repulsive and attractive forces.

Independent variable is something that is changed in a experiment.