Which of the following is a recommended practice for maintaining a motorcycle helmet?

Storing a motorcycle helmet bottom side down is a recommended practice as it helps protect the interior padding and structure of the helmet. This position minimizes the risk of damaging the helmet's protective foam or other components that can be compromised if the helmet is placed on its side or top. Keeping the helmet in this position also prevents dirt or debris from entering through the vents or visor.

Other practices mentioned, such as modifying the retention system, can compromise the helmet's fit and effectiveness in a crash. Exposing the helmet to direct sunlight can degrade the materials and affect its integrity over time. Using paint to customize a helmet can also interfere with its structural integrity and may void the manufacturer's warranty. Therefore, storing it bottom side down is the only choice that supports the helmet's safety and maintenance.