Which requirements describe the papers that must be carried on board an aircraft used for passenger transportation?

The requirement that describes the papers that must be carried on board an aircraft used for passenger transportation is focused on operational safety and regulatory compliance. This set of regulations outlines the necessary documentation that pilots and airlines must have readily available during flight operations.

JAR OPS 1 specifically establishes the rules for commercial air operations, including what documentation should be present on board for passenger flights. This encompasses elements such as the aircraft’s registration, maintenance records, and other operational documents that validate the aircraft's airworthiness and compliance with safety standards. The focus is on ensuring that both the aircraft and its operations meet established safety criteria for transporting passengers.

Other options address different aspects of aviation regulations; for example, JAR-21 pertains to the certification of aircraft and components, JAR-145 relates to maintenance organization approvals, and JAR-45 deals with the continued airworthiness of aircraft, but none of these specifically cover the required papers for passenger transportation like JAR OPS 1 does. Thus, the correct choice directly correlates to the operational requirements for passenger-carrying flights, making it the appropriate answer.