Final answer:
A spirometer is an instrument used to measure the volume of air inhaled and exhaled by the lungs. It is not to be confused with a sphygmomanometer, which measures blood pressure, or a stethoscope used to listen to body sounds.
Step-by-step explanation:
A spirometer is an instrument that measures the volume of air moved with each breath. Other options such as a sphygmomanometer, stethoscope, and plethysmograph are used for different purposes. A sphygmomanometer measures blood pressure and typically consists of an inflatable cuff and a diaphragm gauge. A stethoscope is used to listen to the internal sounds of the body, such as the heartbeat. A plethysmograph is used to measure changes in volume within an organ or whole body (often used to measure blood flow). In contrast, a spirometer is specifically designed to evaluate the function of the lungs by measuring respiratory volumes and capacities, which can help diagnose respiratory diseases or determine the effectiveness of disease treatment.