Final answer:
The situation that could be considered unsafe when unloading a Class 3 vehicle into a hopper is when the hopper is positioned downhill.
Step-by-step explanation:
The situation that could be considered unsafe when unloading a Class 3 vehicle into a hopper is when the hopper is positioned downhill (option c). When the hopper is positioned downhill, gravity can cause the unloading process to become uncontrolled and fast, potentially leading to accidents or damage. On the other hand, options a, b, and d contribute to a safe unloading process. Level ground (option a), a secure and stable hopper (option b), and a controlled and gradual unloading process (option d) all minimize the risk of accidents or mishaps during unloading.