Which of the following features characterizes a complex airplane?

A complex airplane is characterized by several advanced systems that enhance performance and handling characteristics, making it necessary for pilots to have additional training and endorsement. The defining features include retractable landing gear, flaps, and a controllable pitch propeller.

The inclusion of retractable landing gear provides aerodynamic benefits by reducing drag when the gear is retracted, which can improve speed and fuel efficiency. Flaps enhance the aircraft's ability to fly at lower speeds and improve lift during takeoff and landing phases. A controllable pitch propeller allows the pilot to adjust the blade angle, optimizing engine performance for different flight conditions, such as climbing, cruising, or descending.

In contrast, the other options describe aircraft characteristics that do not meet the criteria for complexity. Fixed gear and fixed pitch propellers are indicative of simpler aircraft designs. Single-engine and low-wing layouts, or multi-engine and high-wing configurations, don't inherently imply complexity in terms of operational requirements or systems complexity. These characteristics may apply to numerous aircraft types but do not include the advanced systems that define a complex airplane.