Staying at least one vehicle length back when stopped behind another vehicle on a hill is important for safety reasons. This distance allows for adequate space to maneuver or react if the vehicle in front rolls back or if you need to move in case of an emergency. In particular, on a hill, gravity can cause vehicles to roll backward, especially if the driver in front inadvertently takes their foot off the brake.
By keeping this distance, you ensure that you have a safe buffer zone. In the event the vehicle in front begins to roll back, you have enough space to respond without colliding. It also provides you room to see the road ahead and any changes in traffic that might require your attention.
The other options suggest maintaining greater distances, which, while providing additional safety, may not be necessary in most situations where one vehicle length is sufficient to manage the risk effectively.