The reaction force that propels a rocket forward is called
a. gravity.
b. inertia.
c. velocity.
d. thrust.

The correct answer is letter D. Thrust. The reaction force that propels a rocket forward is called thrust. In physics, thrust is the force which makes an object move. In a car, take for example, the force produced by the engine making the car move forward is the thrust. Another example is in a rocket the thrust of the engines makes the rocket take off.

The reaction force which propels a rocket forward is called thrust - d. 

Thrust is the force which gets created when a rocket's engine gets started up and all of the fuel starts getting burned up. This bundled up energy helps rockets fly up into the sky.