Final answer:
The supine position, where the person lies on their back, is associated with the greatest decrease in Functional Residual Capacity (FRC) due to increased pressure on the diaphragm, which limits lung expansion.
Step-by-step explanation:
The changes in patient position that are associated with the greatest decrease in Functional Residual Capacity (FRC) is the supine position. Being in a supine position, which means lying on the back with the face upwards, can reduce FRC because the weight of the abdominal contents exerts pressure on the diaphragm, which limits its movement and reduces the volume of the lungs at the end of passive expiration. This contrasts with the sitting upright position, where FRC is typically greater due to less pressure on the diaphragm and better lung expansion.