What is the definition of "airworthiness"?

The definition of "airworthiness" refers specifically to the ability of an aircraft to operate safely in flight. This concept encompasses a wide range of considerations, including the structural integrity of the aircraft, the functionality and reliability of its systems and components, and adherence to relevant regulatory and operational standards. An aircraft deemed airworthy is one that is capable of being flown safely, minimizing risks to both the crew and passengers.

The other options, while related to aircraft operation and maintenance, do not encapsulate the essence of what it means for an aircraft to be airworthy. For example, the cost of an aircraft's maintenance relates to financial aspects rather than the aircraft's safety and operational capacity. Similarly, aerodynamic efficiency focuses on performance characteristics rather than safety and reliability. Lastly, while the availability of spare parts is important for maintenance and repair, it doesn’t directly indicate whether an aircraft can be safely operated in flight. Thus, the correct answer emphasizes the core principle of airworthiness as it pertains to flight safety.