Answer:
Answer C : person on an elevator going down
Explanation:
We look for a change in state of motion in order to have a net force different from zero in the system. The state of motion doesn't change in any of the examples a, b, or d. But there is a change in the state of motion in case c where initially the person and elevator where in static equilibrium, and later the elevator started going down and accelerating from zero speed to another non zero speed.
