What document must a pilot carry while operating an aircraft?

A pilot is required to carry a valid pilot's license and a medical certificate while operating an aircraft. The pilot's license serves as proof of the pilot's qualifications and authorization to act as pilot-in-command of an aircraft, ensuring they have met the necessary training, knowledge, and skills mandated by aviation regulations. Additionally, the medical certificate demonstrates that the pilot meets the required medical standards to ensure their fitness to fly, which is crucial for maintaining safety in aviation operations.

The other options, while relevant in certain contexts, do not satisfy the legal requirements for a pilot to operate an aircraft. The aircraft registration certificate is essential for the legal ownership and identification of the aircraft but is not a document the pilot must hold personally. A logbook containing flight records is important for tracking a pilot's experience and currency but is not required to be carried while flying. A passenger manifest is related to commercial operations and is not a requirement for a private pilot flying with or without passengers.