Final answer:
Decreased oxygen levels lead to a lower affinity of hemoglobin for oxygen, which increases the amount of oxygen unloaded by hemoglobin to peripheral tissues.
Step-by-step explanation:
A decrease in pH, an increase in carbon dioxide levels, or an increase in temperature can all lead to more oxygen being unloaded by hemoglobin to peripheral tissues. This is explained by the oxygen-hemoglobin saturation/dissociation curve, which shifts to the right under these conditions, indicating a lower affinity of hemoglobin for oxygen.
Therefore, the correct factor that would increase the amount of oxygen unloaded by hemoglobin into peripheral tissues if decreased is oxygen levels (Answer D). This is because at lower oxygen levels, hemoglobin's affinity for oxygen decreases, facilitating more oxygen to be released to the tissues.