What defines a retractable landing gear system?

A retractable landing gear system is defined by its ability to fold up into the aircraft during flight. This design is primarily employed to reduce drag, which can enhance the aircraft's aerodynamic efficiency and improve fuel performance. When the landing gear is retracted, it minimizes the surface area that disrupts airflow around the aircraft, allowing for better speed and fuel consumption.

The other choices do not align with the characteristics of retractable landing gear. For instance, if the landing gear remains extended during flight, it would create significant drag, counteracting the efficiency benefits of retracting the gear. While retractable landing gear can be found in larger aircraft, it is not exclusive to them, as many smaller aircraft also utilize this system. A fixed landing gear system, which cannot change position, represents a different design entirely and does not offer the aerodynamic advantages that a retractable system does.