Which maintenance level ensures the continued flying integrity of airframes and flight systems during operational periods?

The correct answer is C, as the D-level maintenance is specifically focused on ensuring the long-term integrity and airworthiness of aircraft, which includes major repairs and overhauls of airframes and flight systems. This level of maintenance is typically performed at specialized facilities with the capability to restore an aircraft to a fully operational state, dealing with more complex and extensive repairs that cannot be handled at the lower levels.

D-level maintenance is critical in maintaining the safety and reliability of airframes and flight systems, especially under the rigors of operational demands. It typically includes processes such as structural repairs, replacement of major components, and comprehensive upgrades, which are essential for the ongoing operational integrity of the aircraft.

In contrast, O-level maintenance is generally more routine and focuses on day-to-day support, while I-level maintenance serves as an intermediate stage that encompasses more complex repairs than O-level but is not to the extent of D-level. Maintenance level one may not specifically refer to any recognized designation among aviation maintenance levels, contributing to the clarity around why D-level is the best choice for this question.