What defines a semi-trailer?

A semi-trailer is specifically designed to rest partially on the towing vehicle, which allows it to be connected to a tractor unit using a fifth wheel coupling. This design distinguishes semi-trailers from other types of trailers that are fully supported by their own wheels. The fact that a semi-trailer relies on the towing vehicle for part of its support enables efficient weight distribution and maneuverability, making them particularly well-suited for transporting large loads over long distances.

The other options describe characteristics that do not define semi-trailers. A trailer that can operate independently refers to a full trailer, which is supported entirely by its own wheels and does not require a towing vehicle for stability. A trailer designed for cargo transport only could apply to many types of trailers, including those that are not semi-trailers. Lastly, a trailer that carries no weight would not fulfill the intended purpose of any trailer, especially in the context of those used for commercial transportation.