Which system requires secure restraint against opening without rider access?

The correct choice is the restraint system. This system is designed to ensure the safety of riders by preventing access to the operating mechanism without proper authorization or access controls. Secure restraints are vital in any setting where equipment can be hazardous if accessed inappropriately.

In the context of attractions or rides, the restraint system is essential because it protects users from engaging with the equipment in an unsafe manner, thereby reducing the risk of accidents or injuries. Having secure restraints ensures that only authorized personnel, such as operators or maintenance staff, can access the mechanisms, thus maintaining safety compliance and operational integrity.

While other options such as ride unit, operator guidelines, and safety protocols are related to overall safety and operation, they do not specifically point to the need for secure restraint against unauthorized access. The ride unit refers to the ride itself, which may include various systems and components, but it does not focus solely on the safety measures of restricting access. Operator guidelines provide instructions but do not directly implement security measures, and safety protocols typically encompass broader practices that promote safety but might not focus solely on access control. Thus, the emphasis on securing a restraint system is pivotal to ensuring safety and regulatory compliance.