Which item is not an indication of mandatory snowmobiling equipment?

The correct choice highlights that a Navigation GPS is not categorized as mandatory equipment for snowmobiling. While it can be a valuable tool for enhancing safety and ensuring proper navigation in the wilderness, it is not a legal requirement for operating a snowmobile.

Mandatory equipment typically includes items that are essential for safety and compliance with regulations. A helmet protects the rider's head, working headlights ensure visibility and safety during low-light conditions, and a visible registration plate is necessary for legal identification of the snowmobile on the trails. These items enhance the safety of the operator and other individuals on the trails. Therefore, while a GPS may be used to aid navigation, it does not fall under the category of legally required equipment for snowmobiling.