Which maintenance action is mainly focused on ensuring an aircraft continues to operate safely without major changes?

Preventive maintenance is the correct choice because it comprises routine procedures and tasks that are performed to preserve the condition of an aircraft and ensure that it operates safely without introducing significant modifications or changes to its structure, systems, or performance. This type of maintenance includes regular inspections, lubrication, adjustments, and replacements of parts that are subject to wear and tear.

The essence of preventive maintenance lies in its proactive approach; it aims to identify and address potential issues before they develop into significant problems that could jeopardize safety. By ensuring that maintenance actions are performed regularly and appropriately, overall aircraft reliability and safety can be maintained.

In contrast, major repairs typically involve significant alterations to the structural integrity or systems of the aircraft. Annual inspections are thorough checks required for regulatory compliance but may not necessarily focus on routine upkeep. Significant alterations refer to modifications that change the aircraft's performance or capabilities, which goes beyond the scope of simply ensuring safe operation. These types of actions may introduce complexities that preventive maintenance directly seeks to avoid.