In which position does a resident lie flat on her back to maintain a correct body position?

The correct choice is supine. In this position, a resident lies flat on her back, which is critical for maintaining proper alignment of the spine and distributing weight evenly to prevent pressure sores. This position can also support the respiratory and circulatory systems by allowing for optimal lung expansion and blood flow.

It is important to distinguish the supine position from the others mentioned. Fowler's position involves lying back but at an angle, typically used for individuals who may have difficulty breathing or need to eat comfortably. The prone position involves lying face down, which can hinder airway access and is less commonly used in many care settings. The lateral position entails lying on the side, which can be beneficial for certain conditions but does not maintain the straight alignment of the body that is characteristic of the supine position.