Final answer:
The velocity of the rock when it returns to the original height it was thrown from is 8 m/s.
Step-by-step explanation:
In this scenario, the rock is thrown upwards from a tower with an initial velocity of 8 m/s. When the rock returns to its original height, its velocity is the same as its initial velocity, which is 8 m/s. In this case, if the rock is thrown upward with an initial velocity of 8 m/s, the velocity of the rock when it returns to the original height it was thrown from will also be 8 m/s, but with a downward direction (assuming downward is negative). This follows from the conservation of mechanical energy and the symmetric nature of the motion when only gravity is acting on the object. Therefore, the answer is option b. 8 m/s.