What is the primary role of a passenger cabin on an aircraft?

The primary role of a passenger cabin on an aircraft is to provide space for passengers to sit. It is designed specifically to accommodate individuals during flights, ensuring their comfort and safety. The cabin is outfitted with seating arrangements, safety features, and amenities that cater to passengers' needs, creating an environment for travel.

While the other options address important components of an aircraft, they do not pertain to the primary function of the passenger cabin. For instance, navigation equipment is located in the cockpit rather than the cabin, and the engines are housed in specific compartments outside the passenger area. Additionally, while luggage storage is a function of the aircraft, it typically occurs in designated cargo spaces or overhead compartments, rather than being the main feature of the cabin itself. Thus, the correct answer focuses clearly on the primary purpose of the passenger cabin.