What is defined as the cabin of an aircraft?

The cabin of an aircraft is specifically defined as the interior area where passengers are seated. This space is designed for passenger comfort during flights, often featuring seating arrangements, overhead compartments for belongings, and sometimes facilities such as restrooms and galley areas for refreshments.

While the cockpit is essential for pilot operations, it is a separate region from the passenger cabin. The external structure of the airplane pertains to the shell and framework that protects and holds the aircraft's components, but it does not refer to the areas where passengers are located. The area designated for cargo and equipment is also distinct from the cabin, as it typically involves the lower deck or separate compartments used specifically for luggage and freight storage. Thus, the correct choice emphasizes the area specifically reserved for passengers, highlighting its purpose and design within the overall aircraft structure.