Final answer:
The federal regulation, 14 CFR 91.23 mandates that a copy of the lease agreement must be kept on board when a large aircraft is operated under part 91. This is to ensure clarity regarding the aircraft's operational control and regulatory compliance.
Step-by-step explanation:
The statement that a copy of the lease must be carried on board a large aircraft operated under part 91 is true. Per Federal Aviation Regulations under part 91, specifically 14 CFR 91.23, it is mandated that a duplicate of the lease agreement or any contract by which the aircraft is operated must be kept on the aircraft when it is in operation. This ensures that there is a clear understanding of the operational control of the aircraft and compliance with the regulations.