What is the primary purpose of a glider's control surfaces?

The primary purpose of a glider's control surfaces is to control the aircraft's attitude, direction, and speed, making this the correct choice. Control surfaces, such as ailerons, elevators, and rudders, play a crucial role in maneuvering the aircraft in three dimensions.

Attitude refers to the orientation of the glider in the air; for instance, elevators are used to pitch the aircraft up or down, affecting its climb or descent. Direction is managed through ailerons and the rudder, allowing the pilot to bank or turn the glider, which is essential for navigating and maintaining desired flight paths. Speed control is also vital, as the pilot may want to adjust the glider's angle of attack to optimize lift and drag, thus affecting airspeed.

While fuel efficiency and passenger comfort are relevant to many aircraft, they do not pertain primarily to the control surfaces in the context of gliders. Instead, their main function revolves around the pilot's ability to steer and maintain control during flight.