Final answer:
A pulse oximeter is the instrument that measures oxygen saturation by measuring fractional hemoglobin.
Step-by-step explanation:
The instrument that measures oxygen saturation by measuring fractional hemoglobin is called a pulse oximeter. It works by sending two different wavelengths of light through a thin part of the body, typically the fingertip, and measuring the light as it exits. The device then calculates the percent saturation of hemoglobin based on the amount of light absorbed by the oxygenated hemoglobin.