An aircraft's integral fuel tank is?

An integral fuel tank is designed as a part of the aircraft's structure itself. This means that the fuel tank is integrated into the airframe, typically within the wings or fuselage, and utilizes the space within the structural components to hold fuel. This design helps reduce weight by eliminating the need for a separate, detachable fuel tank and allows for more efficient use of available space in the aircraft.

In contrast, other design options involve either external attachments, such as external tanks that can be mounted on the outside of the aircraft, or separate compartments that do not utilize the aircraft's existing structure effectively. The choice of making fuel tanks integral to the structure enhances the overall aerodynamic efficiency and contributes to the overall strength of the airframe, making it a preferable design in many aircraft.