Which one of the following would be most affected by a decrease in the affinity of hemoglobin for oxygen?
a. partial pressure of o2 in systemic arterial blood
b. partial pressure of co2 in systemic arterial blood
c. percent saturation of hemoglobin with o2 in systemic arterial blood
d. percent saturation of hemoglobin with o2 in systemic venous blood

C. percent saturation of hemoglobin with O2 in systemic arterial blood

Explanation:

Hemoglobin is a protein  that contains 4 heme groups that have iron (Fe) as its central atom, which is the binding site for the oxygen molecule (the oxygen binds with the iron atom), so each hemoglobin molecule can bind up to 4 oxygen molecules. This oxygenated blood is the arterial blood, which becomes bright red because of this binding, and is the blood that carries that oxygen to all the tissues.

Sometimes, conditions like temperature, pH, or CO2 concentration can affect the structure of the hemoglobin, and that can make it more difficult for oxygen to bind to heme group, so in those cases that would have a direct effect on the percentage of saturated hemoglobin (hemoglobin binded with oxygen).