Final answer:
Air resistance decelerates the rocket's motion as it ascends.
Step-by-step explanation:
Air resistance has a decelerating effect on a rocket's motion as it ascends.
When a rocket is launched straight up, it experiences an upward force from the expelled gas that overcomes the force of gravity and accelerates it upward. However, air resistance acts in the opposite direction, slowing down the rocket's upward motion. As the rocket continues to ascend, air resistance becomes stronger, eventually causing the rocket to decelerate and eventually come to a stop.
Therefore, the correct answer is (b) It decelerates the rocket downward.