Final answer:
The velocity of the ball as it falls back down and reaches the starting height after being thrown at +15m/s and staying in the air for 1.53 seconds is -15m/s, indicating a downward direction.
Step-by-step explanation:
If a ball is thrown at +15m/s, and the ball stays in the air for a total of 1.53 seconds, the final velocity of the ball as it falls down and reaches the starting height would be -15m/s. This is because in the absence of air resistance, the acceleration due to gravity (which is 9.8 m/s² downward) affects the ball equally during its ascent and descent. When the ball is thrown upwards at 15m/s, it decelerates at 9.8 m/s² until it reaches the highest point. Its velocity decreases by 9.8 m/s every second, and thus it takes slightly more than 1.53s for the initial upward velocity to be reduced to zero and then to equal the magnitude of the initial velocity but in the downward direction as it falls back down. The correct answer is -15 m/s (Option C), indicating the velocity is in the opposite direction of the initial throw.