No person shall load or transport any loose materials on or over the highways unless what condition is met?

The correct answer emphasizes the importance of properly enclosing loose materials during transport to ensure safety on the highways. When the bed carrying the load is completely enclosed, it minimizes the risk of loose materials spilling onto the road. This is crucial not only for the safety of other drivers but also for preventing damage to the roadway and limiting environmental hazards.

In contrast, while visibility and securing loads are important considerations, they do not directly address the primary concern of preventing loose materials from becoming a hazard during transport. For example, if a load is secured with ropes but is not contained properly, it could still overflow. Similarly, having the load fully visible does not guarantee that it is stable or safe for transit. Ensuring the load is within an enclosed bed addresses the core issue of material safety during transport on highways.