Final answer:
The legally mandatory equipment to ride a motorcycle is a helmet. Other items like reflective jackets, GPS navigation, and knee pads are not typically required by law but can enhance safety and convenience.
Step-by-step explanation:
The mandatory equipment to legally ride a motorcycle varies by jurisdiction, but a universally required piece of safety equipment is a helmet. This means the correct answer to the question, 'To legally ride a motorcycle, which equipment is mandatory?' is option c. Helmet. Reflective jackets, GPS navigation, and knee pads, although beneficial for safety and convenience, are not typically required by law for motorcycle operation. Wearing a helmet is crucial as it significantly reduces the risk of head injuries in the event of an accident.