Which of the following actions is a violation of transportation code?

Holding onto a moving motor vehicle while riding a bicycle is a violation of transportation code primarily due to the significant danger this action poses. It creates a hazardous situation not only for the cyclist but also for drivers and pedestrians. The act of a cyclist gripping a vehicle can lead to serious injuries or fatalities if there is any sudden motion or stop. This behavior disrupts normal traffic flow and compromises the safety regulations in place for all road users.

In contrast, the other options, while potentially unsafe or discouraged in certain contexts, do not universally constitute violations of the transportation code. For instance, riding without a helmet may not be illegal in all jurisdictions, and handling a bicycle in a pedestrian lane can be permissible under specific conditions. Riding on sidewalks may also have different regulations depending on the local laws and the area in which the bicyclist is traveling. Therefore, the action of holding onto a moving vehicle stands out as a clear breach of safety laws meant to protect everyone on the road.