Which of the following is not required for bicycles operated at night?

Bicycles operated at night must adhere to specific safety requirements to ensure visibility and safety on the road. Among these, a red reflector on the rear and a white light on the front are essential, as they help other road users see the cyclist and gauge their approach. The red reflector is crucial for reflecting headlights from vehicles behind, while the white light is necessary for visibility from the front.

A bell can be a useful signaling device, but it is not a legal requirement for nighttime operation, making it the one not mandated by regulations. Similarly, wearing a helmet is strongly encouraged for safety, but it is not a legal requirement for nighttime riding in many jurisdictions. Therefore, the absence of a bell is what distinguishes it as not required for bicycles operating at night.