Identify the force that propels a rocket?

The fundamental force that propels a rocket is electromagnetism.

What happens is that fuel particles are given a high speed by some chemical reaction (the burning) and these are ejected from the rocket at the bottom. Because momentum has to be conserved, and these particles have a net downward momentum (the particles moving upwards collide with the rocket), the rocket gains a net upward momentum.

The collision part is where electromagnetism comes in; it is the force that is responsible for objects being solid. It is what keeps you from falling through your office chair right now!