Which of the following is a legal requirement for cyclists in many areas?

Wearing a helmet is a legal requirement for cyclists in many areas due to safety regulations aimed at reducing head injuries in the event of a fall or collision. Helmets are designed to absorb impact energy and protect the wearer’s head, making them a critical piece of safety gear for cyclists. The law encourages or mandates helmet use to promote safer riding practices, especially among young cyclists and in locations with high traffic.

Other options do not conform to safety standards or legal mandates. Riding without brakes would compromise a cyclist's ability to stop safely, using a loud horn is not a standard requirement for cyclists, and riding against traffic poses significant risks, as it can lead to accidents by reducing the cyclist's reaction time to oncoming vehicles. Thus, the emphasis on helmet use reflects a commitment to safety in cycling.