What causes an airplane to slip when entering a turn?

A slip occurs when an airplane is not coordinated in a turn and is primarily affected by the balance between the bank angle and the amount of rudder input. When a pilot banks the aircraft to initiate a turn without using an adequate amount of rudder to counteract the yaw induced by the bank, the airplane can end up slipping towards the inside of the turn. This imbalance happens because the lift vector shifts with the bank, causing an excess of lateral force that isn't fully countered by the rudder, resulting in a loss of sideways traction and control.

The correct option highlights this key aspect of aircraft handling during turns. When proper rudder input is not applied in relation to the bank angle, the airplane will start to yaw towards the outside of the turn, leading to a slip condition. Maintaining coordinated flight is essential to ensure that the airplane turns effectively without slipping or skidding, which enhances safety and aircraft performance during maneuvers.