Integral fuel tanks are best described as:

Integral fuel tanks are best described as being formed by the aircraft structure. This means that these tanks are not standalone containers but are built into the wings or fuselage of the aircraft itself. The design and integration of these tanks help to save weight and maximize the available space for fuel storage, enhancing the overall fuel efficiency and aerodynamic qualities of the aircraft.

By utilizing the structural components of the aircraft to also serve as fuel storage, integral tanks contribute to a more streamlined design and reduce the need for additional external fuel tanks. This integration also helps to lower the center of gravity and maintain the aircraft's structural integrity.

In contrast, other types of fuel tanks might be made from specific materials, be removable for servicing, or function independently from the aircraft's structure, but those characteristics do not define integral fuel tanks. The focus on the tank's role as part of the aircraft's overall structure is what makes option C the most accurate description.