**Mopeds** are two, three, or four-wheeled vehicles with low-power engines and a limited maximum speed. When there is no specific lane for them, they must travel on the **shoulder** of the road, provided it is passable and sufficient. This is because mopeds, like other low-speed vehicles, are required to use the shoulder to avoid interfering with faster traffic on the main roadway.

- The shoulder is a part of the road intended for certain slower vehicles, such as mopeds, to ensure traffic safety and flow.
- If the shoulder is not passable or sufficient, mopeds may use only the essential part of the lane, but this is an exception.

Therefore, the correct option is that mopeds must travel on the shoulder if it is passable and sufficient.