Which of the following is NOT a form in which action required by an AD can be presented?

Action required by an Airworthiness Directive (AD) is typically presented in several specific forms, primarily focusing on maintaining the safety and integrity of aircraft. Among the options listed, a reduction in maintenance personnel does not pertain to the technical aspects of aircraft operation or safety.

Inspections, design modifications, and changes in operating procedures are all crucial elements outlined in ADs to ensure that all aircraft are maintained in compliance with safety standards and regulations, often in response to identified safety hazards or defects. For instance, inspections are mandated to check the integrity of aircraft components; design modifications may be required to rectify issues with the original design; and changes in operating procedures could be necessary to enhance safety protocols.

In contrast, reducing maintenance personnel does not address the physical aspects of aircraft safety or operational procedures. Instead, it relates to the administrative or logistical side of maintenance and does not influence the technical requirements necessary for compliance with an AD. Therefore, it is not a recognized form of action mandated by an AD, making it the right choice for this question.