Which document must be on board at all times during flight?

The aircraft's registration certificate is a crucial document that must be on board at all times during flight. This certificate serves as proof that the aircraft is officially registered with the appropriate aviation authority, which confirms the legal ownership of the aircraft. It is necessary for compliance with legal regulations, as it verifies that the aircraft meets the requirements for registration, ensuring it is recognized as airworthy and legally allowed to operate. Without this document, operate the aircraft could face legal ramifications, as it may not satisfy the aviation authority’s regulations regarding the documentation required to be carried during flight.

The other options, while relevant to aircraft operation and safety, do not have the same legal necessity regarding their presence in the cockpit during a flight. The pilot's logbook, for instance, is important for tracking flight experience and currency but is not a requirement to carry on the aircraft. Similarly, the aircraft's maintenance history is essential for understanding the aircraft's upkeep but does not need to be on board. Airworthiness directives, important for maintaining the aircraft's safety standards, might also not be required on every flight as they inform maintenance procedures rather than serve as proof of compliance with registration.