Final answer:
The thrown ball's speed, when added to the parachute's speed, totals 24 m/s for a stationary observer. Therefore, the correct answer is (c) 24 m/s.
Step-by-step explanation:
If a parachute is moving east at a speed of 20 m/s and a ball is thrown from it at a speed of 4 m/s, then the speed of the ball thrown out of the moving parachute, as observed from an external stationary frame of reference, would be the vector sum of the two speeds. Since the speeds are in the same direction, the speeds are simply additive. Therefore, the speed of the ball as observed by a stationary observer would be:
Speed of parachute + speed of ball = 20 m/s + 4 m/s = 24 m/s.
The correct answer to this question is (c) 24 m/s.