What causes an airplane to turn?

The correct answer identifies that an airplane turns primarily due to the horizontal component of lift produced when the wing is banked. When a pilot wants to initiate a turn, they will bank the aircraft by tilting the wings in the direction of the desired turn. This banking action changes the orientation of the lift vector.

In straight and level flight, lift acts vertically upward while weight acts downward. However, when the aircraft is banked, the lift vector tilts, and part of the lift now acts horizontally toward the direction of the turn. This horizontal component of lift is what enables the aircraft to change its flight path and turn effectively. The greater the degree of bank, the larger the horizontal component of lift, thereby allowing for tighter turns.

Other factors like throttle reduction, rudder deflection, or thrust increase can influence the maneuvering of the plane or help in coordinating turns, but they are not the primary reason for initiating the turn itself. The banking of the wings and the resultant horizontal component of lift is fundamental to the mechanics of turning in flight. Thus, this reasoning solidifies why the horizontal component of lift due to wing banking is essential for an airplane to turn.