What is pulse oximetry and what does it help evaluate?

Pulse oximetry is a non-invasive method of monitoring pulse rate and the percentage of oxygenated hemoglobin in arterial blood.
Pulse oximetry helps evaluate the lungs' ability to supply oxygen and is an indirect indicator of the partial pressure of oxygen in the blood (PO2) available for tissue perfusion.
There is no 1:1 correlation between SpO2 and PO2.
All of the above.