Which type of airport is primarily used for scheduled passenger-carrying operations with air carriers holding more than 9 seats?

The correct choice highlights Part 139 airports, which are defined by specific federal regulations that govern the operation of commercial service airports in the United States. These airports are primarily used for scheduled passenger-carrying operations and are required to meet certain safety and operational standards set forth by the Federal Aviation Administration (FAA).

Part 139 airports accommodate air carriers that operate aircraft with more than 9 passenger seats, making them essential hubs for commercial aviation. They undergo regular inspections and must implement strict safety measures, including emergency response plans and other operational protocols, to ensure passenger safety. These requirements help establish the airport as a reliable and safe facility for airline operations.

Other types of airports mentioned, such as Class II, Joint-Use, and Shared-Use airports, have different operational focuses or characteristics that do not specifically fit the description of primarily serving scheduled passenger operations for larger air carriers. For example, Class II airports may serve smaller aircraft or have different regulatory requirements, and Joint-Use and Shared-Use airports often involve multiple entities, including military and civilian operations, rather than being dedicated solely to large commercial passenger operations.