What is the definition of a complex airplane?

A complex airplane is defined primarily by the presence of retractable landing gear and a controllable pitch propeller, which distinguishes it from simpler aircraft. These features enhance performance, provide greater efficiency, and ensure a smoother operation, particularly in terms of takeoff and landing. The retractable landing gear reduces drag while in flight, contributing to improved fuel efficiency and overall aircraft performance. A controllable pitch propeller allows the pilot to adjust the angle of the propeller blades, optimizing engine performance and efficiency in various flight conditions.

The other choices describe attributes that do not align with the definition of a complex airplane. Fixed landing gear and manual flaps are characteristics of simpler aircraft, multi-engine aircraft can be either complex or simple depending on the design (as not all multi-engine aircraft have retractable gears or controllable pitch propellers), and aerobatic capabilities pertain to an aircraft's design and maneuverability rather than its complexity related to landing gear and propeller type.