Describe the "school bus stop" sign.

The "school bus stop" sign is designed specifically to communicate that the bus is actively loading or unloading students. When this sign is displayed, it signals to other drivers that they must take extra caution and be prepared to stop, as children may be entering or exiting the bus. This is a crucial safety measure to protect young students who may be crossing the street or approaching the bus.

The correct option emphasizes the significance of the bus's activities in relation to student safety, which is the primary purpose of the sign. It is important for drivers to be aware of this sign to prevent accidents and ensure that children can safely board or leave the bus. Other options do not accurately reflect the specific purpose of the school bus stop sign. For instance, a sign that indicates stopping at all times would suggest a continuous halt rather than a focus on loading and unloading. Similarly, a sign indicating a route or a pedestrian crossing serves different functions and does not pertain to the immediate actions of a school bus with respect to student safety.