Which flight control surface is primarily responsible for changing the aircraft's pitch?

The elevator is the flight control surface that primarily changes the aircraft's pitch. Located on the trailing edge of the horizontal stabilizer, the elevator moves up and down to control the angle of the airplane's nose relative to its flight path. When the pilot pulls back on the control stick or yoke, the elevator deflects upward, causing the tail of the aircraft to drop and the nose to rise, resulting in an increase in pitch. Conversely, when the elevator is deflected downward, the pitch decreases, and the nose goes down.

In contrast, ailerons are responsible for controlling roll by adjusting the lift on each wing, and the rudder is mainly used for yaw control, helping to coordinate turns and counteract adverse yaw. Flaps serve to increase lift during takeoff and landing but do not significantly affect pitch under normal flight conditions. Hence, the elevator is distinctly designed to manage pitch, making it the correct answer.